The sum of the present ages of a father and his son is 60 years. Five years ago, father's age was four times the age of the son, so now the son's age will be

Correct Answer: (C) 15

Let father's and son's present ages be $F$ and $S$. Given $F + S = 60$. Five years ago, $F - 5 = 4(S - 5)$, which simplifies to $F - 4S = -15$. Subtracting the two equations gives $5S = 75$, so the son's present age is $S = 15$ years.