The average age of three boys is 15 years and their ages are in proportion…

Solution:

Total age of the three boys = average × number of boys = 15 × 3 = 45 years.

Let the common multiplier be x, so the ages are 3x, 5x and 7x.

Sum of ages = (3 + 5 + 7)x = 15x = 45, therefore x = 45 ÷ 15 = 3.

Youngest age = 3x = 3 × 3 = 9 years.