Son is 19.8 years old; father is 79.2 years old.

Call the age of the father F, and the son's age S.

You know the combined age of the father and son:

(equation 1)

And you know that the father is four times older than the son:

(equation 2)

So you've just got simultaneous equations.

Rearrange and substitute and you arrive at two more equations, one of which tells you the age of the father, and the other tells you the age of the son:

...and...