### The average age of three boys is 15 years. If their ages are in ratio 3 : 5 : 7, the age of the youngest boy is.

A. 21 years

B. 18 years

C. 15 years

D. 9 years

Sum of ages of three boys = 45 years Now, $\begin{array}{l}\text{(3x + 5x + 7x) = 45}\\ \text{⇒ 15x = 45}\\ \text{⇒ x = 3}\\ \text{So,}\\ \text{age of youngest boy = 3x}\\ \text{= 3 × 3}\\ \text{= 9 years}\end{array}$