Question

The father is 60 years old and his son is half his age. how old was the boy when his father was 4 times his age?

Answer 1

Let y= years ago when dad's age was 4 times son's age

60-y=4(30-y)
60-y=120-4y
-y+4y=120-60
3y=60
y=60/3
y= 20 years ago

Then 30-20= 10 yrs. old, the boys age when his dad was 40 years old.