The difference between the ages of a man and his son is 20. If the man is 4 times older than his son

Let $M$ be the age of the man.
Let $S$ be the age of the son.

From the first statement, "The difference between the ages of a man and his son is 20":

From the second statement, "the man is 4 times older than his son":

Substitute $4S$ for $M$ in equation (1):

Divide both sides by 3:

Substitute the value of $S$ back into equation (2):

The age of the son is $\frac{20}{3}$ years, and the age of the man is $\frac{80}{3}$ years.