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Dear student,
 

Class Interval	5-10	10-15	15-20	20-25	25-30	30-35	35-40
frequency	6	10	18	30	15	12	10
c.f	6	16	34	64	79	91	101

Q1 = size of ${\left(\frac{n}{4}\right)}^{th}$ item = ${\left(\frac{101}{4}\right)}^{th}$ = 25.25^th item
Class interval for Q1 = 15-20

Q1 = l₁ + $\frac{{\displaystyle \frac{n}{4}} - cf}{f}\times i$
Q1 = 15 + $\frac{25.25 -16}{18} \times$ 5
Q1 = 17.57

Q3 = size of 3 ${\left(\frac{n}{4}\right)}^{th}$ item =  3 ${\left(\frac{101}{4}\right)}^{th}$ = 75.75^th item
Class interval for Q3 = 25-30

Q3 = l₁ + $\frac{3 \left({\displaystyle \frac{n}{4}}\right)- cf}{f}\times i$
Q3 = 25 + $\frac{75.75 -64}{15} \times$ 5
​Q3 = 28.92

Quartile deviation = $\frac{Q3 - Q1}{2}$ = $\frac{11.35}{2}$ = 5.68