The average age of 3 boys is 15 years and their ages are in proportions 3:5:7. The age in year of the younger boys is:

Suppose,
Age of 3 Boys=3x+5x and 7z
According to Questions:
(3x+5x+7y)/3=15
15x/3=15
15x=15*3
15x=45
x=45/15
x=3
According to Question, the age of younger boys=3x
3x=3(3)=9