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