In a class there were 9 boys and some girls. In a test, the mean score obtained by the boys was 13 while that obtained by the girls was 15 . If the overall average was 14.28 , what was the total number of students in the class?

Correct option is D

Given:

- Number of boys = 9
- Mean score of boys = 13
- Mean score of girls = 15
- Overall average score = 14.28

Formula Used:
Overall average = $$\frac{(Total \ score \ of \ boys + Total \ score \ of \ girls) }{ Total \ number \ of students}$$​​
Solution:
Let number of girls be x.
Total students = 9 + x
Total score of boys = 9 × 13 = 117
Total score of girls = 15 × x
Using overall average formula:
​$$14.28 = \frac{(117 + 15x) }{(9 + x)}$$​​
=> 14.28(9 + x) = 117 + 15x
=> 128.52 + 14.28x = 117 + 15x
=> 128.52 - 117 = 15x - 14.28x
=> 11.52 = 0.72x
=> x = 11.52 / 0.72 = 16

Total number of students = 9 + 16 = 25

Hence, the total number of students in the class is 25.