### The average age of all the students of a class is 18 years. The average age of boys of the class is 20 years and that of the girls is 15 years. If the number of girls in the class is 20, then find the number of boys in the class.

A. 15 B. 30 C. 45 D. 50 Answer: Option B

### Solution(By Apex Team)

Let the number of boys in the class be x Then, $\begin{array}{l}\text{⇒ 18(x + 20) = 20x + 15 × 20}\\ \text{⇒ 18x + 360 = 20x + 300}\\ \text{⇒ 2x = 60}\\ \text{⇒ x = 30}\end{array}$

A. 20
B. 21
C. 28
D. 32

A. 18
B. 20
C. 24
D. 30

A. 10 years
B. 10.5 years
C. 11 years
D. 12 years