A father is 60 years old and his son is half his age. How old was the boy when his father was four times his age?

If the father is 60 years old and the son is half his age, the son is currently 30 years old. When the father was four times the son's age, he would have been 40 years old (since 40 is four times 10). At that time, the son was 10 years old. Thus, the boy was 10 years old when his father was four times his age.