If the current son's age is x, the current age of the father is
.
In 20 years they will be, respectively,
and
years old, and we know that the father will be 3 times as old as his son:

The quadratic equation has solutions

So, the two solutions are

We clearly can't accept negative values as solution for a problem involving the age of a person, so the only feasible solution is x=8, and we deduce that the father's age is 8 squared, i.e. 64.