I am currently twice the age of my son. Ten years ago I was three times his age. How old are we both?
Let F be my age and S be my son's age.
F = 2S
F - 10 = 3(S - 10) giving
F = 3S - 20
So 2S = 3S - 20 giving
3S = 2S + 20 and
S = 20
So my son is 20 and I'm 40, and ten years ago I was 30 and he was 10.