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?

Now, father is 60 years old and his son is half his age
so son's age is 60/2 = 30 years old
let x = the number of years back then when his father was four times his age
4(30 - x) = 30
120 - 4x = 30
 4x = 80
   x = 20

so 20 years back then
son's age = 30 - 20 = 10
father's age = 60 - 20 = 40

answer
the son's age was 10 when his father was 40 (four times his age)