average age of three boys =25 years, their ages are in the proportion 3:5:7
Lets us assume their ages =3x,5x,7x
As we know, an average is the sum of a list of numbers divided by the number of numbers in the list.
Average age of three boys =3x+5x+7x3
25=15x3
25=5x
x=5
The age of the youngest boy =3x=3×5=15 years