Question 225315
"a father is 3 times as old as his son" ___ f = 3s


"15 years ago, the father was 9 times as old as his son was then" ___ f-15 = 9(s-15)


substituting ___ 3s - 15 = 9(s-15)


distributing ___ 3s - 15 = 9s - 135


adding 135-3s ___ 120 = 6s


dividing by 6 ___ 20 = s


substituting ___ f = 3(20) = 60