Question

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?

Answer 1

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 answer: The boy was 10 (and his father was 40).