mathschallenge.net

Problem

When I was 14 years old my father was 42 years old, which was three times my age. Now he is twice my age, how old am I?

Solution

Let $f$ be the father's age and his child's age be $c$.

As the father is 42 14 = 28 years older than the child, $f$ = $c$ + 28.

At the moment $f$ = 2$c$, hence 2$c$ = $c$ + 28, and so $c$ = 28. That is, the father's child is now 28 years old.