Question

The average age of three boys is 15. If there age are in ratio 3 : 5 : 7, then what is the age of youngest boy?

Answer 1

Add x to the ratios then add them that is 3x + 5x + 7x = 15x
15x = 45
x = 3

Therefore the ages are 9, 15 and 21 and the yougest must be 9 yrs.old.

Mark brainliest or thanks if it helped you :D

Answer 2

Let their ages be 3x, 5x, and 7x. 

Average = $\frac{3x + 5x + 7x}{3} = \frac{15x}{3} = 5x = 15 \\ \\ x = \frac{15}{5} = 3$ 

Age of youngest boy = 3x = 3 × 3 = 9. 

9 is the answer. 

Hope this will be helpful.