x = the age of his son now, then the age of the father is 4x.
In 20 years,
son's age = x + 20
father's age = 4x + 20
solve the equation for x:
4x + 20 = 2(x + 20)
4x + 20 = 2x + 40
2x = 20
x = 10
So the age of his son now is 10, and the age of the the father now is 40.
