Afather 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, the son is 30 (half the age).

From here, we can tell that father's age= son's age+30. We can now create the system of equations as follows.
f= s+30
f=4s

Because both define f, put the two equations together.
4s=s+30
3s=30
s=10

Final The boy was 10 (and his father was 40).