The age of the older of two boys is thrice that of the younger. 9 years ago, it was five times that of the younger. Find the present age of each.

Correct option is D

Given:
Let the Age of the younger boy be x years,
The age of older boy is = 3x years.
9 years ago, their age
Younger boy = x-9 years.
Older boy = 3x-9 years.
Solution:
(3x-9)=5(x-9)
3x - 9 = 5x – 45
-5x + 3x = -45 + 9
-2x = -36
2x = 36
x = 18
So, the present age of older and younger boys are 54 years and 18 years respectively.
Alternate Method:
As per the given question,
Older younger
Their present age ratio - 3 : 1
Ages 9 years ago - 5 :  1
Difference of their age ratios should be same in both the condition.
So, the new ration be
Older younger
Their present age ratio - 6 :  2
Ages 9 years ago - 5 : 1
Difference of present age – old age = 1 unit
1 unit → 9 year,
So,
6 unit → 54 years
2 unit → 18 years