A father is now three times as old as his son. Five years back, he was four times as old as his son. The age of the son (in years) is

Explanation:-

Let son's age be x years. Then, father's age = (3x) years.
Five years ago, father's age = (3x - 5) years and son's age = (x - 5) years.
So, 3x - 5 = 4 (x - 5) ⇔ 3x - 5 = 4x - 20 ⇔ x = 15.