RD Sharma 2017 Solutions for Class 9 Math Chapter 24 Measures Of Central Tendency are provided here with simple step-by-step explanations. These solutions for Measures Of Central Tendency are extremely popular among class 9 students for Math Measures Of Central Tendency Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the RD Sharma 2017 Book of class 9 Math Chapter 24 are provided here for you for free. You will also love the ad-free experience on Meritnation’s RD Sharma 2017 Solutions. All RD Sharma 2017 Solutions for class 9 Math are prepared by experts and are 100% accurate.

Page No 24.10:

Question 17:

The mean weight per student in a group of 7 students is 55 kg. The individual weights of 6 of them (in kg) are 52, 54, 55, 53, 56 and 54. Find the weight of the seventh student.

Answer:

Given that the mean weight (in kg) of 7 students is 55 kg. Let us denote their weights by.

Ifbe the mean of the n observations, then we have

Hence the sum of the weights of 7 students is

The individual weights (in kg) of 6 of them are 52, 54, 55, 53, 56 and 54.

Let the weight of the remaining student is x.

According to this information the sum of their weights is

Hence we have

Thus the weight of the seventh student is.

Page No 24.10:

Question 18:

The mean weight of 8 numbers is 15. If each number is multiplied by 2, what will be the new mean?

Answer:

Let us denote the 8 numbers by. Their mean is 15.

Ifbe the mean of the n numbers, then we have

Therefore the sum of the numbers is

If each numbers is multiplied by 2, then the numbers becomes.

The mean of the new numbers is

Note that the new mean is equal to the old mean multiplied by 2.

Page No 24.10:

Question 19:

The mean of 5 numbers is 18. If one number is excluded, their mean is 16. Find the excluded number.

Answer:

Given that the mean of 5 numbers is 18. Let us denote the numbers by.

Ifbe the mean of the n observations, then we have

Hence the sum of 5 numbers is

If one number is excluded then the mean becomes 16 and the total numbers becomes 4.

Let the number excluded be x.

After excluding one number the sum becomesand then the mean is

But it is given that after excluding one number the mean becomes 16.

Hence we have

Thus the excluded number is.

Page No 24.10:

Question 20:

The mean of 200 items was 50, Later on, it was discovered that the two items were misread as 92 and 8 instead of 192 and 88, Find the correct mean.

Answer:

Given that the mean of 200 items is 50. Let us denote the 200 items by.

Ifbe the mean of the n observations, then we have

Hence we have

This sum is incorrect. We have to find the correct sum

It was discovered that the two items were misread as 92 and 8 instead of 192 and 88. So, 92 and 8 are must be replaced by 192 and 8 respectively.

To get the correct sum at first we have to remove the incorrect items and then add the correct items.

Therefore the correct sum is

Thus, the correct mean is

Page No 24.10:

Question 21:

Find the values of n and X in each of the following cases:

(i) t=1n(xi-12)=-10 and t=1n(xi-3)=62

(ii) t=1n(xi-10)=30 and t=1n(xi-6)=150

Answer:

Ifbe the mean of the n observations, then we have

(i) The given two equations are

The equations can be rewritten as

We have to solve the above equations forand.

Subtracting the first equation from the second equation, we have

Substitute the value of n in the first equation, we have

(ii) The given two equations are

The equations can be rewritten as

We have to solve the above equations forand.

Subtracting the first equation from the second equation, we have

Substitute the value of n in the first equation, we have

Page No 24.10:

Question 22:

The sum of the deviations of a set of n values x1, x2,...., xn, measured from 15 and −3 are −90 and 54 respectively. Find the value of n and mean.

Answer:

Ifbe the mean of the n observations, then we have

Given that the sums of the deviations of a set of n valuesmeasured from 15 and -3 are -90 and 54 respectively. So that we get two equations

These equations can be rewritten as

We have to solve the above equations forandunknowns.

Subtracting the first equation from the second equation, we have

Substitute the value of n in the first equation, we have

Page No 24.10:

Question 23:

Find the sum of the deviations of the variate values 3, 4, 6, 7, 8, 14 from their mean.

Answer:

Ifbe the mean of the n observations, then we have

The given 6 variate values are 3, 4, 6, 7, 8 and 14. Let us denote the values by X1, X2, ..., X6. Their mean value is given by

X=16i=16 Xi   =163+4+6+7+8+14   =16×42=7

The sum of the deviations of the variate values from their mean is

i=16Xi-X=i=16Xi-7=i=16Xi-i=167=6X-6×7=6×7-6×7=0

Hence the proof is complete.

Page No 24.10:

Question 24:

If X¯ is the mean of the ten natural numbers x1, x2, x3,..., x10, show that
(x1-X¯)+(x2-X¯)+...+(x10-X¯) = 0.
 

Answer:

Ifbe the mean of the n observations, then we have

Given thatis the mean of the 10 natural numbers. So, we have

We have to show that

That is to show that the sum of the deviations of the 10 natural numbers from their mean is 0.

We have

Hence the proof is complete.



Page No 24.14:

Question 1:

Calculate the mean for the following distribution:
 

x: 5 6 7 8 9
f: 4 8 14 11 3

Answer:

The given distribution in tabulated form is

Prepare the following frequency table of which the first column consists of the values of the variate and the second column the corresponding frequencies. Multiply the frequency of each row with the corresponding values of variable to obtain the third column containing.

Find the sum of all entries in the second and third column to obtain N andrespectively. Therefore,

Hence the mean is

Page No 24.14:

Question 2:

Find the mean of the following data:
 

x: 19 21 23 25 27 29 31
f: 13 15 16 18 16 15 13

Answer:

The given distribution in tabulated form is

Prepare the following frequency table of which the first column consists of the values of the variate and the second column the corresponding frequencies. Multiply the frequency of each row with the corresponding values of variable to obtain the third column containing.

Find the sum of all entries in the second and third column to obtain N andrespectively. Therefore,

Hence the mean is

Page No 24.14:

Question 3:

The mean of the following data is 20.6. Find the value of p.
 

x: 10 15 p 25 35
f: 3 10 25 7 5

Answer:

The given distribution in tabulated form is

We have to find the value of p using the information that the mean of the distribution is 20.6.

Prepare the following frequency table of which the first column consists of the values of the variate and the second column the corresponding frequencies. Multiply the frequency of each row with the corresponding values of variable to obtain the third column containing.

Find the sum of all entries in the second and third column to obtain N andrespectively. Therefore,

The mean is

Hence, we have



Page No 24.15:

Question 4:

If the mean of the following data is 15, find p.
 

x: 5 10 15 20 25
f: 6 p 6 10 5

Answer:

The given data in tabulated form is

We have to find the value of p using the information that the mean of the data is 15.

Prepare the following frequency table of which the first column consists of the values of the variate and the second column the corresponding frequencies. Multiply the frequency of each row with the corresponding values of variable to obtain the third column containing.

Find the sum of all entries in the second and third column to obtain N andrespectively. Therefore,

The mean is

Hence, we have

Page No 24.15:

Question 5:

Find the value of p for the following distribution whose mean is 16.6.
 

x: 8 12 15 p 20 25 30
f: 12 16 20 24 16 8 4

Answer:

The given distribution in tabulated form is

We have to find the value of p using the information that the mean of the distribution is 16.6.

Prepare the following frequency table of which the first column consists of the values of the variate and the second column the corresponding frequencies. Multiply the frequency of each row with the corresponding values of variable to obtain the third column containing.

Find the sum of all entries in the second and third column to obtain N andrespectively. Therefore,

The mean is

Hence, we have

Page No 24.15:

Question 6:

Find the missing value of p for the following distribution whose mean is 12.58.
 

x: 5 8 10 12 p 20 25
f: 2 5 8 22 7 4 2

Answer:

The given distribution in tabulated form is

We have to find the value of p using the information that the mean of the distribution is 12.58.

Prepare the following frequency table of which the first column consists of the values of the variate and the second column the corresponding frequencies. Multiply the frequency of each row with the corresponding values of variable to obtain the third column containing.

Find the sum of all entries in the second and third column to obtain N and respectively. Therefore,

The mean is

Hence, we have

Page No 24.15:

Question 7:

Find the missing frequency (p) for the following distribution whose mean is 7.68.
 

x: 3 5 7 9 11 13
f: 6 8 15 p 8 4

Answer:

The given distribution in tabulated form is

We have to find the value of p using the information that the mean of the distribution is 7.68.

Prepare the following frequency table of which the first column consists of the values of the variate and the second column the corresponding frequencies. Multiply the frequency of each row with the corresponding values of variable to obtain the third column containing.

Find the sum of all entries in the second and third column to obtain N and respectively. Therefore,

The mean is

Hence, we have

Page No 24.15:

Question 8:

Find the value of p, if the mean of the following distribution is 20.
 

x: 15 17 19 20+p 23
f: 2 3 4 5p 6

Answer:

The given data in tabulated form is

We have to find the value of p using the information that the mean of the data is 20.

Prepare the following frequency table of which the first column consists of the values of the variate and the second column the corresponding frequencies. Multiply the frequency of each row with the corresponding values of variable to obtain the third column containing.

Find the sum of all entries in the second and third column to obtain N and respectively. Therefore,

The mean is

Hence, we have

If p is negative then the 4th frequency becomes negative. But frequency can’t be negative. Hence the possible value of p is 1

Page No 24.15:

Question 9:

Find the mean of the following distribution:

 

x: 10 12 20 25 35
f: 3 10 15 7 5

Answer:

The given distribution in tabulated form is

Prepare the following frequency table of which the first column consists of the values of the variate and the second column the corresponding frequencies. Multiply the frequency of each row with the corresponding values of variable to obtain the third column containing. Find the sum of all entries in the second and third column to obtain N andrespectively.

Find the sum of all entries in the second and third column to obtain N and respectively. Therefore,

Hence the mean is

Page No 24.15:

Question 10:

Candidates of four schools appear in a mathematics test. The data were as follows:
 

Schools No. of Candidates Average Score
I
II
III
IV
60
48
Not available
40
75
80
55
50

If the average score of the candidates of all the four schools is 66, find the number of candidates that appeared from school III.

Answer:

Let the number of candidates appeared from school III is.

Then the given data can be tabulated as

The row of x denotes the average scores and the row of f denotes the number of candidates. We have to find the value of p using the information that the average score of all the four schools is 66.

Prepare the following frequency table of which the first column consists of the average scores and the second column the number of candidate (frequencies). Multiply the frequency of each row with the corresponding average scores to obtain the third column containing.

Find the sum of all entries in the second and third column to obtain N and respectively. Therefore,

The mean is

Hence, we have

Hence the number of candidates appeared from school III is.

Page No 24.15:

Question 11:

Five coins were simultaneously tossed 1000 times and at each toss the number of heads were observed. The number of tosses during which 0, 1, 2, 3,4 and 5 heads were obtained are shown in the table below. Find the mean number of heads per toss.
 

No. of heads per toss No. of tosses
0
1
2
3
4
5
38
144
342
287
164
25
Total 1000

Answer:

The given data can be tabulated in the form

The row of x denotes the number of heads per toss and the row of f denotes the number of tosses.

Prepare the following frequency table of which the first column consists of the number of heads and the second column the number of tosses (frequencies). Multiply the frequency of each row with the corresponding number of heads to obtain the third column containing.

Find the sum of all entries in the second and third column to obtain N (already given in the question) and respectively. Therefore,

Hence the mean number of heads per toss is



Page No 24.16:

Question 12:

Find the missing frequencies in the following frequency distribution if it is known that the mean of the distribution is 50.
 

x: 10 30 50 70 90
f: 17 f1 32 f2 19

Total 120.

Answer:

The given distribution in tabulated form is

We have to find the value of missing frequencies f1 and f2, using the information that the mean of the distribution is 50 and the total frequency is 120.

Prepare the following frequency table of which the first column consists of the values of the variate and the second column the corresponding frequencies. Multiply the frequency of each row with the corresponding values of variable to obtain the third column containing.

Find the sum of all entries in the second and third column to obtain N and respectively. Therefore,

The mean is

Hence, we have two equations

Adding them we get

Putting the value of f1 in the second equation we get



Page No 24.18:

Question 1:

Find the median of the following data (1-8)

83, 37, 70, 29, 45, 63, 41, 70, 34, 54

Answer:

The given data is 83, 37, 70, 29, 45, 63, 41, 70, 34 and 54.

Arranging the given data in ascending order, we have

29, 34, 37, 41, 45, 54, 63, 70, 70, 83

Here, the number of observation, which is an even number.

Hence, the median is

Page No 24.18:

Question 2:

133, 73, 89, 108, 94, 104, 94, 85, 100, 120

Answer:

The given data is 133, 73, 89, 108, 94, 104, 94, 85, 100 and 120.

Arranging the given data in ascending order, we have

73, 85, 89, 94, 94, 100, 104, 108, 120, 133

Here, the number of observation, which is an even number.

Hence, the median is

Page No 24.18:

Question 3:

31, 38, 27, 28, 36, 25, 35, 40

Answer:

The given data is 31, 38, 27, 28, 36, 25, 35 and 40.

Arranging the given data in ascending order, we have

25, 27, 28, 31, 35, 36, 38, 40

Here, the number of observation, which is an even number.

Hence, the median is

Page No 24.18:

Question 4:

15, 6, 16, 8, 22, 21, 9, 18, 25

Answer:

The given data is 15, 6, 16, 8, 22, 21, 9, 18, and 25.

Arranging the given data in ascending order, we have

6, 8, 9, 15, 16, 18, 21, 22, 25

Here, the number of observation, which is an odd number.

Hence, the median is

Page No 24.18:

Question 5:

41, 43, 127, 99, 71, 92, 71, 58, 57

Answer:

The given data is 41, 43, 127, 99, 71, 92, 71, 58 and 57.

Arranging the given data in ascending order, we have

41, 43, 57, 58, 71, 71, 92, 99, 127

Here, the number of observation, which is an odd number.

Hence, the median is

Page No 24.18:

Question 6:

25, 34, 31, 23, 22, 26, 35, 29, 20, 32

Answer:

The given data is 25, 34, 31, 23, 22, 26, 35, 29, 20 and 32.

Arranging the given data in ascending order, we have

20, 22, 23, 25, 26, 29, 31, 32, 34, 35

Here, the number of observation, which is an even number.

Hence, the median is

Page No 24.18:

Question 7:

12, 17, 3, 14, 5, 8, 7, 15

Answer:

The given data is 12, 17, 3, 14, 5, 8, 7 and 15.

Arranging the given data in ascending order, we have

3, 5, 7, 8, 12, 14, 15, 17

Here, the number of observation, which is an even number.

Hence, the median is

Page No 24.18:

Question 8:

92, 35, 67, 85, 72, 81, 56, 51, 42, 69

Answer:

The given data is 92, 35, 67, 85, 72, 81, 56, 51, 42 and 69.

Arranging the given data in ascending order, we have

35, 42, 51, 56, 67, 69, 72, 81, 85, 92

Here, the number of observation, which is an even number.

Hence, the median is

Page No 24.18:

Question 9:

Numbers 50, 42, 35, 2x + 10, 2x − 8, 12, 11, 8 are written in descending order and their median is 25, find x.

Answer:

The given data in descending order is

50, 42, 35, 2x+10, 2x-8, 12, 11, 8

Here, the number of observation, which is an even number.

Hence, the median is

But, it is given that the median is 25. Hence, we have

Page No 24.18:

Question 10:

Find the median of the following observations: 46, 64, 87, 41, 58, 77, 35, 90, 55, 92, 33. If 92 is replaced by 99 and 41 by 43 in the above data, find the new median?

Answer:

The given data is 46, 64, 87, 41, 58, 77, 35, 90, 55, 92 and 33.

Arranging the given data in ascending order, we have

33, 35, 41, 46, 55, 58, 64, 77, 87, 90, 92

Here, the number of observation, which is an odd number.

Hence, the median is

If 92 and 41 are replaced by 99 and 43 respectively, then the given data becomes 46, 64, 87, 43, 58, 77, 35, 90, 55, 99 and 33.

Arranging the new data in ascending order, we have

33, 35, 43, 46, 55, 58, 64, 77, 87, 90, 99

Here, the number of observation, which is an odd number.

Hence, the new median is

Page No 24.18:

Question 11:

Find the median of the following data: 41, 43, 127, 99, 61, 92, 71, 58, 57 If 58 is replaced by 85, what will be the new median

Answer:

The given data is 41, 43, 127, 99, 61, 92, 71, 58 and 57.

Arranging the given data in ascending order, we have

41, 43, 57, 58, 61, 71, 92, 99, 127

Here, the number of observation, which is an odd number.

Hence, the median is

Ifis replaced by 85, then the given data become 41, 43, 127, 99, 61, 92, 71, 85 and 57.

Arranging the new data in ascending order, we have

41, 43, 57, 61, 71, 85, 92, 99, 127

Here, the number of observation, which is an odd number.

Hence, the new median is

Page No 24.18:

Question 12:

The weights (in kg) of 15 students are : 31, 35, 27, 29, 32, 43, 37, 41, 34, 28, 36, 44, 45, 42, 30. Find the median. If the weight 44 kg is replaced by 46 kg and 27 kg by 25 kg, find the new median.

Answer:

The weights (in kg) of 15 students are 31, 35, 27, 29, 32, 43, 37, 41, 34, 28, 36, 44, 45, 42 and 30.

Arranging them in ascending order, we have

27, 28, 29, 30, 31, 32, 34, 35, 36, 37, 41, 42, 43, 44, 45

Here, the number of observation, which is an odd number.

Hence, the median is

If the weights 44kg and 27kg are replaced by 46kg and 25kg respectively, then the given data becomes 31, 35, 25, 29, 32, 43, 37, 41, 34, 28, 36, 46, 45, 42 and 30.

Arranging the new data in ascending order, we have

25, 28, 29, 30, 31, 32, 34, 35, 36, 37, 41, 42, 43, 45, 46

Here, the number of observation, which is an odd number.

Hence, the new median is

Page No 24.18:

Question 13:

The following observations have been arranged in ascending order. If the median of the data is 63, find the value of x:

23, 32, 48, 50, x, x+2, 72, 78, 84, 95

Answer:

The given data in ascending order is

29, 32, 48, 50, x, x+2, 72, 78, 84, 95

Here, the number of observation, which is an even number.

Hence, the median is

But, it is given that the median is 63. Hence, we have



Page No 24.20:

Question 1:

Find out the mode of the following marks obtained by 15 students in a class:

Marks: 4, 6, 5, 7, 9, 8, 10, 4, 7, 6, 5, 9, 8, 7, 7.

Answer:

Given that the marks obtained by 15 students are 4, 6, 5, 7, 9, 8, 10, 4, 7, 6, 5, 9, 8, 7 and 7.

Make the following frequency table.

Since the value 7 occurs maximum number of times, that is, 4. Hence, the mode value is 7.

Page No 24.20:

Question 2:

Find the mode from the following data:

125, 175, 225, 125, 225, 175, 325, 125, 375, 225, 125

Answer:

The given data is 125, 175, 225, 125, 225, 175, 325, 125, 375, 225, and 125.

Make the following frequency table.

Since the value 125 occurs maximum number of times, that is, 4. Hence, the mode value is

Page No 24.20:

Question 3:

Find the mode for the following series:

7.5, 7.3, 7.2, 7.2, 7.4, 7.7, 7.7, 7.5, 7.3, 7.2, 7.6, 7.2

Answer:

The given series is 7.5, 7.3, 7.2, 7.2, 7.4, 7.7, 7.7, 7.5, 7.3, 7.2, 7.6 and 7.2.

Make the following frequency table.

Since the value 7.2 occurs maximum number of times, that is, 4. Hence, the modal value is.

Page No 24.20:

Question 4:

Find the mode of the following data in each case:

(i) 14, 25, 14, 28, 18, 17, 18, 14, 23, 22, 14, 18

(ii) 7, 9, 12, 13, 7, 12, 15, 7, 12, 7, 25, 18, 7

Answer:

(i) The given data is 14, 25, 14, 28, 18, 17, 18, 14, 23, 22, 14 and 18.

Make the following frequency table.

Here occurs maximum number of times (that is 4). Hence, the modal value is 14.

(ii) The given data is 7, 9, 12, 13, 7, 12, 15, 7, 12, 7, 25, 18 and 7.

Make the following frequency table.

Since the value 7 occurs maximum number of times, that is, 5. Hence, the modal value is 7.

Page No 24.20:

Question 5:

The demand of different shirt sizes, as obtained by a survey, is given below:
 

Size: 38 39 40 41 42 43 44 Total
Number of persons
(wearing it):
26 39 20 15 13 7 5 125
Find the modal shirt sizes, as observed from the survey.

Answer:

The demand of different shirt sizes is given as

Here x denotes the size of the shirts and f denotes the number of peoples (frequency) wearing it.

The value 39 occur maximum number of times, that is, 39. Hence, the modal shirt size is 39.



Page No 24.21:

Question 1:

If the ratio of mean and median of a certain data is 2:3, then find the ratio of its mode and mean

Answer:

Given that the ratio of mean and median of a certain data is 2:3. That is,

We know that

Page No 24.21:

Question 2:

If the ratio of mode and median of a certain data is 6 : 5, then find the ratio of its mean and median.

Answer:

Given that the ratio of mode and median of a certain data is 6:5. That is,

We know that

Page No 24.21:

Question 3:

If the mean of x + 2, 2x + 3, 3x + 4, 4x + 5 is x + 2, find x.

Answer:

The given data is x+2, 2x+3, 3x+4, 4x+5. They are four in numbers.

The mean is

But, it is given that the mean is x+2. Hence, we have

Page No 24.21:

Question 4:

The arithmetic mean and mode of a data are 24 and 12 respectively, then find the median of the data.

Answer:

Given that the arithmetic mean and mode of a data are 24 and 12 respectively. That is,

We have to find median

We know that

Page No 24.21:

Question 5:

If the difference of mode and median of a data is 24, then find the difference of median and mean.

Answer:

Given that the difference of mode and median of a data is 24. That is,

We have to find the difference between median and mean

We know that

Page No 24.21:

Question 6:

If the median of scores x2,x3,x4,x5 and x6 (where x > 0) is 6, then find the value of x6.

Answer:

Given that the median of the scores, whereis 6. The number of scores n is 5, which is an odd number. We have to find

Note that the scores are in descending order. Hence the median is

But, it is given that the median is 6. Hence, we have

Page No 24.21:

Question 7:

If the mean of 2, 4, 6, 8, x, y is 5, then find the value of x + y.

Answer:

The given data is. They are 6 in numbers.

The mean is

But, it is given that the mean is 5. Hence, we have

Page No 24.21:

Question 8:

If the mode of scores 3, 4, 3, 5, 4, 6, 6, x is 4, find the value of x.

Answer:

The given data is.

The mode is the value which occur maximum number of times, that is, the mode has maximum frequency. If the maximum frequency occurs for more than 1 value, then the number of mode is more than 1 and is not unique.

Here it is given that the mode is 4. So, x must be 4, otherwise it contradicts that the mode is 4. Hence

Page No 24.21:

Question 9:

If the median of 33, 28, 20, 25, 34, x is 29, find the maximum possible value of x.

Answer:

The given data is. The total number of values is, is an even number. Hence the median depends on theobservation andobservation.

Since we have to find the maximum possible value of x.So we must put it in the 4th position when ordering in ascending order.

Arranging the data in ascending order, we have

Hence, the median is

Here it is given that the median is 29. So, we have

Page No 24.21:

Question 10:

If the median of the scores 1, 2, x, 4, 5 (where 1 < 2 < x < 4 < 5) is 3, then find the mean of the scores.

Answer:

The given data is 1, 2, x, 4 and 5. Since, the given data is already in ascending order.

Here, the number of observation, which is an odd number.

Hence, the median is

Here, it is given that the median is 3. Hence, we have.

The mean is

Page No 24.21:

Question 1:

Mark the correct alternative in each of the following:
Which one of the following is not a measure of central value?

(a) Mean

(b) Range

(c) Median

(d) Mode

Answer:

We know that mean, median and mode are all measures of central tendency.

Hence, the correct choice is (b).

Page No 24.21:

Question 2:

The mean of n observations is X. If k is added to each observation, then the new mean is

(a) X

(b) X + k

(c) X − k

(d) kX

Answer:

Let us take n observations.

Ifbe the mean of the n observations, then we have

Add a constant k to each of the observations. Then the observations becomes

Ifbe the mean of the new observations, then we have

Hence, the correct choice is (b).

Page No 24.21:

Question 3:

The mean of n observations is X. If each observation is multiplied by k, the mean of new observations is

(a) kX¯

(b) X¯k

(c) X¯+k

(d) X¯-k

Answer:

Let us take n observations.

Ifbe the mean of the n observations, then we have

Multiply a constant k to each of the observations. Then the observations becomes

Ifbe the mean of the new observations, then we have

Hence, the correct choice is (a).

Page No 24.21:

Question 4:

The mean of a set of seven numbers is 81. If one of the numbers is discarded, the mean of the remaining numbers is 78. The value of discarded number is

(a) 98

(b) 99

(c) 100

(d) 101
 

Answer:

Given that the mean of 7 numbers is 81. Let us denote the numbers by.

Ifbe the mean of the n observations, then we have

Hence the sum of 7 numbers is

If one number is discarded then the mean becomes 78 and the total numbers becomes 6.

Let the number discarded is x.

After discarding one number the sum becomesand then the mean is

But it is given that after discarding one number the mean becomes 78.

Hence we have

Thus the excluded number is. So, the correct choice is (b).



Page No 24.22:

Question 5:

For which set of numbers do the mean, median and mode all have the same value?
(a) 2, 2, 2, 2, 4

(b) 1, 3, 3, 3, 5

(c) 1, 1, 2, 5, 6

(d) 1, 1, 1, 2, 5

Answer:

For the data 2, 2, 2, 2, 4 of 5 numbers, we have

Since, 2 occurs maximum number of times,

For the data 1, 3, 3, 3, 5 of 5 numbers, we have

Since, 3 occurs maximum number of times,

Hence, the correct choice is (b).

Note that if it happens that the result is not true for the second data then we must follow the same procedure for the other data’s.

Page No 24.22:

Question 6:

For the set of numbers 2, 2, 4, 5 and 12, which of the following statements is true?

(a) Mean = Median

(b) Mean > Mode

(c) Mean > Mode

(d) Mode = Median

Answer:

For the data 2, 2, 4, 5, 12 of 5 numbers, we have

Since, 2 occurs maximum number of times,

So, the correct choice is (b).

Page No 24.22:

Question 7:

If the arithmetic mean of 7, 5, 13, x and 9 is 10, then the value x is

(a) 10

(b) 12

(c) 14

(d) 16

Answer:

The given data is 7, 5, 13, x and 9. They are 5 in numbers.

The mean is

But, it is given that the mean is 10. Hence, we have

Hence, the correct choice is (d).

Page No 24.22:

Question 8:

If the mean of five observations x, x+2, x+4, x+6, x+8, is 11, then the mean of first three observations is

(a) 9

(b) 11

(c) 13

(d) none of these

Answer:

The given data is x, x + 2,  x + 4, x + 6 and x + 8. They are 5 in numbers.

The mean is

But, it is given that the mean is 11. Hence, we have

Then the first three observations are 7, 7 + 2, 7+4, that is, 7, 9, 11. Their mean is

Hence, the correct choice is (a).

Page No 24.22:

Question 9:

Mode is

(a) least frequent value

(b) middle most value

(c) most frequent value

(d) none of these

Answer:

We know that, mode is the observation which occur maximum number of times.

Hence, the correct choice is (c).

Page No 24.22:

Question 10:

The following is the data of wages per day : 5, 4, 7, 5, 8, 8, 8, 5, 7, 9, 5, 7, 9, 10, 8
The mode of the data is

(a) 7

(b) 5

(c) 8

(d) 10

Answer:

The given data is 5, 4, 7, 5, 8, 8, 8, 5, 7, 9, 5, 7, 9, 10 and 8.

Make the following frequency table.

Since the values 5 and 8 occurs in the data maximum number of times, that is, 4. Hence, the modal value is 5 and 8. In this case the mode is not unique.

Hence, the correct options are (b) and (c).

Page No 24.22:

Question 11:

The empirical relation between mean, mode and median is

(a) Mode = 3 Median − 2 Mean

(b) Mode = 2 Median − 3 Mean

(c) Median = 3 Mode − 2 Mean

(d) Mean = 3 Median − 2 Mode

Answer:

The relation between mean, median and mode is

Hence, the correct option is (a).

Page No 24.22:

Question 12:

The median of the following data : 0, 2, 2, 2, −3, 5, −1, 5, −3, 6, 6, 5, 6 is

(a) 0

(b) −1.5

(c) 2

(d) 3.5

Answer:

The given data is 0, 2, 2, 2, -3, 5, -1, 5, 5, -3, 6, 6, 5 and 6.

Arranging the given data in ascending order, we have

-3, -3, -1, 0, 2, 2, 2, 5, 5, 5, 5, 6, 6, 6

Here, the number of observation, which is an even number.

Hence, the median is

So, the correct choice is (d).

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Question 13:

The mean of a, b, c, d and e is 28. If the mean of a, c, and e is 24, What is the mean of b and d?

(a) 31

(b) 32

(c) 33

(d) 34

Answer:

Given that the mean of a, b, c, d and e is 28. They are 5 in numbers.

Hence, we have

But, it is given that the mean of a, c and e is 24. Hence, we have

Then, we have

Hence, the mean of b and d is.

Hence, the correct choice is (d).

Page No 24.22:

Question 14:

The algebraic sum of the deviations of a set of n values from their mean is

(a) 0

(b) n − 1

(c) n

(d) n + 1

Answer:

Ifbe the mean of the n observations, then we have

Letbe the mean of n values. So, we have

The sum of the deviations of n valuesfrom their meanis

Hence the correct choice is (a).

Page No 24.22:

Question 15:

A, B, C are three sets of values of x:

(a) A: 2, 3, 7, 1, 3, 2, 3
(b) 7, 5, 9, 12, 5, 3, 8
(c) 4, 4, 11, 7, 2, 3, 4

Which one of the following statements is correct?

(a) Mean of A = Mode of C

(b) Mean of C = Median of B

(c) Median of B = Mode of A

(d)Mean, Median and Mode of A are equal.
 

Answer:

For the data A: 2, 3, 7, 1, 3, 2, 3 of 7 numbers, we have

Arranging the data A in ascending order, we have

A: 1, 2, 2, 3, 3, 3, 7

Since, 3 occurs maximum number of times,

Hence, the correct choice is (d).

Note that if it happens that the result is not found in the first step then we must follow the same procedure for the other data’s.



Page No 24.9:

Question 1:

If the height of 5 persons are 140 cm, 150 cm, 152 cm, 158 cm and 161 cm respectively, find the mean height.

Answer:

Given that the heights of 5 persons are 140 cm, 150 cm, 152 cm, 158 cm and 161 cm, respectively.

We have to find the mean of their heights.

Remember the definition of mean of n values x1, x2 xn is

Therefore the mean height of the 5 persons is

Page No 24.9:

Question 2:

Find the mean of 994, 996, 998, 1002, 1000.

Answer:

The given 5 numbers are 994, 996, 998, 1002 and 1000, respectively.

We have to find their mean.

Remember the definition of mean of n values x1, x2 xn is

Therefore the mean of the given 5 numbers is

Page No 24.9:

Question 3:

Find the mean of first five natural numbers.

Answer:

The first 5 natural numbers are 1, 2, 3, 4 and 5.

We have to find their mean.

Remember the definition of mean of n values x1, x2 xn is

Therefore the mean of the first 5 natural numbers is

Page No 24.9:

Question 4:

Find the mean of all factors of 10.

Answer:

All the factors of 10 are 1, 2, 5, and 10. They are 4 in numbers.

We have to find the mean of the all factors of 10.

Remember the definition of mean of n values x1, x2 xn is

Therefore the mean of the all factors of 10 is

Page No 24.9:

Question 5:

Find the mean of first 10 even natural numbers.

Answer:

The first 10 even natural numbers are 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20. They are 10 in numbers.

We have to find the mean of the first 10 even natural numbers.

Remember the definition of mean of n values x1, x2 xn is

Therefore the mean of the first 10 even natural numbers is

Page No 24.9:

Question 6:

Find the mean of x, x+2, x+4, x+6, x+8.

Answer:

We have to find the mean of, and. They are 5 in numbers.

Remember the definition of mean of n values x1, x2 xn is

Therefore the required mean is

Page No 24.9:

Question 7:

Find the mean of first five multiples of 3.

Answer:

The first five multiples of 3 are 3, 6, 9, 12, and 15. They are 5 in numbers.

We have to find the mean of first five multiples of 3.

Remember the definition of mean of n values x1, x2 xn is

Therefore the mean of first five multiples of 3 is

Page No 24.9:

Question 8:

Following are the weights  (in kg)  of 10 new born babies in a hospital on a particular day:
3.4, 3.6, 4.2, 4.5, 3.9, 4.1, 3.8, 4.5, 4.4, 3.6. Find the mean X.

Answer:

The weights (in kg) of 10 new born babies are 3.4, 3.6, 4.2, 4.5, 3.9, 4.1, 3.8, 4.5, 4.4, and 3.6. These are 10 in numbers.

We have to find the mean of their weights.

Remember the definition of mean of n values x1, x2 xn is

Therefore the mean of their weights is

Page No 24.9:

Question 9:

The percentage of marks obtained by students of a class in mathematics are : 64, 36, 47, 23, 0, 19, 81, 93, 72, 35, 3, 1. Find their mean.

Answer:

Given that the percentage of marks obtained by students of a class of mathematics are 64, 36, 47, 23, 0, 19, 81, 93, 72, 35, 3, and 1. These are 12 in numbers.

We have to find the mean of their percentage of marks.

Remember the definition of mean of n values x1, x2 xn is

Therefore the mean of their weights is

Page No 24.9:

Question 10:

The number of children in 10 families of a locality are:
2, 4, 3, 4, 2, 0, 3, 5, 1, 1, 5. Find the mean number of children per family.

Answer:

Given that the numbers of children in 10 families are 2, 4, 3, 4, 2, 3, 5, 1, 1, and 5.

We have to find the mean number of children per family.

Remember the definition of mean of n values x1, x2 xn is

Therefore the mean number of children per family is

Page No 24.9:

Question 11:

If M is the mean of x1, x2, x3, x4, x5 and x6, prove that
(x1 − M) + (x2 − M) (x3 − M) (x4 − M) + (x5 − M) + (x6 − M) = 0.

Answer:

Given that the mean of x1, x2, x3, x4, x5, and x6 is M.

We have to prove that

Remember the definition of mean of n values x1, x2 xn is

Therefore, we have

Now,

Hence the proof is complete.

Page No 24.9:

Question 12:

Durations of sunshine (in hours) in Amritsar to first 10 days of August 1997 as reported by the Meteorological Department are given below:
9.6, 5.2, 3.5, 1.5, 1.6, 2.4, 2.6, 8.4, 10.3, 10.9

(i) Find the mean X.

(ii) Verify that i=110 (xi-X¯) = 0

Answer:

Given that the durations of sunshine (in hours) in Amritsar for 10 days are 9.6, 5.2, 3.5, 1.5, 1.6, 2.4, 2.6, 8.4, 10.3, and 10.9.

(i) We have to find their mean.

Remember the definition of mean of n values x1, x2 xn is

Therefore the mean duration of sunshine is

(ii) We have to prove that

We have

Hence the proof is complete.

Page No 24.9:

Question 13:

Explain, by taking a suitable example, how the arithmetic mean alters by (i) adding a constant k to each term, (ii) subtracting a constant k from each them, (iii) multiplying each term by a constant k and (iv) dividing each term by a non-zero constant k.

Answer:

Let us take n observations.

Ifbe the mean of the n observations, then we have

(i) Add a constant k to each of the observations. Then the observations becomes

Ifbe the mean of the new observations, then we have

Let us take an example to understand the above fact.

The first 5 natural numbers are 1, 2, 3, 4, and 5. Their mean is

Add 2 to each of the numbers. Then the numbers becomes 3, 4, 5, 6, and 7. The new mean is

Therefore adding a constant number to each observation the mean increased by that constant

(ii) Subtract a constant k from each of the observations.

Then the observations becomes

Ifbe the mean of the new observations, then we have

Let us take an example to understand the above fact.

The first 5 even natural numbers are 2, 4, 6, 8 and 10. Their mean is

Subtract 1 from each of the numbers. Then the numbers becomes 1, 3, 5, 7, and 9. The new mean is

Therefore subtracting a constant number to each observation the mean decreased by that constant

(iii) Multiply a constant k to each of the observations. Then the observations becomes

Ifbe the mean of the new observations, then we have

Let us take an example to understand the above fact.

The first 5 even natural numbers are 2, 4, 6, 8 and 10. Their mean is

Multiply 2 to each of the numbers. Then the numbers becomes 4, 8, 12, 16, and 20. The new mean is

Therefore multiplying by a constant number to each observation the mean becomes the constant multiplied by the old mean

(vi) Divide a nonzero constant k to each of the observations. Then the observations becomes

Ifbe the mean of the new observations, then we have

Let us take an example to understand the above fact.

The first 5 even natural numbers are 2, 4, 6, 8 and 10. Their mean is

Divide 2 to each of the numbers. Then the numbers becomes 1, 2, 3, 4, and 5. The new mean is

Therefore dividing by a constant number to each observation the mean becomes the old mean divided by the constant

Page No 24.9:

Question 14:

The mean of marks scored by 100 students was found to be 40. Later on it was discovered that a score of 53 was misread as 83. Find the correct mean.

Answer:

Given that the mean of marks scored by 100 students is 40. Let us denote the marks of 100 students by.

Ifbe the mean of the n observations, then we have

Hence we have

This sum is incorrect. We have to find the correct sum

It was found that a score 53 was misread as 83. So, 83 must be replaced by 53.

To get the correct sum at first we have to remove the incorrect entry and then add the correct entry.

Therefore the correct sum is

Thus, the correct mean is

Page No 24.9:

Question 15:

The  traffic police recorded the speed (in km/hr) of 10 motorists as 47, 53, 49, 60, 39, 42, 55, 57, 52, 48. Later on an error in recording instrument was found. Find the overage speed of the motorists if the instrument  recorded 5 km/hr less in each case.

Answer:

Given that the recorded speed (in km/hr) of 10 motorists are 47, 53, 49, 60, 39, 42, 55, 57, 52, and 48. Let us denote the speeds of 10 motorists by.

Ifbe the mean of the n observations, then we have

Hence we have

This mean is incorrect. We have to find the correct mean.

It was found that the instrument recorded 5 km/hr less in each case.

To get the correct we have to add 5 with each entry. Then the correct entries are

Recall that ifbe the mean of the n observationsand if we add a constant k with each of the observations, then the new mean becomes

Therefore the correct mean speed of the motorists is

Page No 24.9:

Question 16:

The mean of five numbers is 27. If one number is excluded, their mean is 25. Find the excluded number.

Answer:

Given that the mean of 5 numbers is 27. Let us denote the numbers by.

Ifbe the mean of the n observations, then we have

Hence the sum of 5 numbers is

If one number is excluded then the mean becomes 25 and the total numbers becomes 4.

Let the number excluded is x.

After excluding one number the sum becomesand then the mean is

But it is given that after excluding one number the mean becomes 25.

Hence we have

Thus the excluded number is.



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