Question

The average age of all the students of a class is 18 yr. The average age of boys of the class is 20 yr and that of the girls is 15 yr. If the number of girls in the class is 20, then what is the number of boys in the class?

• A. 30
• B. 15
• C. 45
• D. 50

Answer 1

Answer: (A)

30

Answer 2

Let x = boys in class and y=girls in class = 20 (given).

$\text{Average} = \frac{\text{Sum of age}}{\text{No. of students}}$

Given Avg. of class is $=\frac{(20x+15(20))}{(20+x)}=18$

On solving, we get
$x=30$
No of boys in class is 30.. So the correct option is (A)