A father is four times as old as his son. In 4 years, the father will be three times as old as the son. How old is each now?

M = 4S

In three years

M + 3 = 3 (S + 3)

So you put the 4S in to substitute for the M.

4S + 3 = 3 (S + 3)

4S + 3 = 3S + 9

S = 6

If the son is 6, the father must be 24.

We can check this by adding three to both ages. Then, the son will be 9 and the father will be 27, which is three times 9.