Afather is 60 yrs old son is half his age .how old was the boy when his father was four times his age

Let f be the fathers age and let s be the sons age

f = 60 s = 30
when f = 45
s = 15 which is only a third if his fathers age

when f = 40
s = 10 which is a quarter of his fathers age

therefore when his father was 4 times his age, the son was 10

It should be a or d i’m not sure