The average age of three boys is 25 years and their ages are in the proportion 3 : 5 : 7. The age of the youngest boy is:

The youngest boy is 15 years old.

Average age 25 for three boys gives total = 25 x 3 = 75 years.

The ages are in ratio 3 : 5 : 7, summing to 15 parts.

One part = 75 / 15 = 5 years.

Youngest = 3 x 5 = 15 years.