the mean mark obtained by 25 student of section A of class X is 47 and that of 35 student of section B is 51 and that of 30 student of section C is 53. find the mean of the marks scored by 90 student.

Section A:

Number of students = 25

Mean marks = 47

We know that, Sum of observations = Mean × Number of observations

∴ Sum of marks obtained by students of section A = 47 × 25 = 1175

Section B:

Number of students = 35

Mean marks = 51

∴ Sum of marks obtained by students of section B = 51 × 35 = 1785

Section C:

Number of students = 30

Mean marks = 53

∴ Sum of marks obtained by students of section C = 53 × 30 = 1590
 

Now, sum of marks obtained by all students of three sections 

= Sum of marks obtained by students of section A + Sum of marks obtained by students of section B + Sum of marks obtained by students of section C

= 1175 + 1785 + 1590 

= 4550

Total number of students = 25 + 35 + 30 = 90

Mean marks scored by 90 students =