Average age of 5 boys is 16 yr,of which that of 4 boys is 16 yr, 3 months. The age of the 5th boys.

Average of 5 boys = 16 yr
sum of age of 5 boys = 16 x 5 = 80 yr
∴ Average age of 4 boys = 16 - yr 3 month
= 16 yr + 3/12 yr = 16 yr + 1/4 yr
Sum of ages of 4 boys = 4 x (16 +1/4) yr
= 64 +1 = 65 yr
∴ Age of 5th boy = (80 - 65) yr = 15 yr