The present age of a father is three times the age of his son. Five years ago, father's age was four times the age of the son. What is the present age of son?

Let us assume that present age of son is P years then his father's present age will be 3P.
According to question,
Five years ago, father's age was four times the age of the son.
Then
4 (P - 5) = 3P - 5
⇒ 4P - 20 = 3P - 5
∴ P = 15 years
The present age of Son is 15 Years.