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 is

The correct answer is (b), 15 years.

Let $f$ be the father’s current age and $s$ be the son’s current age. We are given that $f = 3s$ and $f – 5 = 4(s – 5)$. Substituting the first equation into the second equation, we get $3s – 5 = 4s – 20$. Solving for $s$, we get $s = 15$.

Option (a), 12 years, is incorrect because $12 \times 3 = 36$, which is not the father’s current age. Option (c), 18 years, is incorrect because $18 \times 3 = 54$, which is not the father’s current age. Option (d), 20 years, is incorrect because $20 \times 3 = 60$, which is not the father’s current age.