let 'x' be the present age of Manoj

after 12 years Manoj will be 3 times as old as he was 4 years ago

x + 12 = 3(x - 4)

x + 12 = 3x - 12

3x - 12 = x + 12

3x - x = 12 + 12

2x = 24

x = 24/2

x = 12

Manjos present age is 12 years old.