Let f and s represent the present ages of father and son.
.. f = 4s . . . . the age of the father is 4 times the age of the son
.. (f +20) = 2*(s +20) . . . . in 20 years, the father will only be 2 times the age of the son.
Substituting for f in the second equation, we have
.. 4s +20 = 2(s +20)
.. 2s = 20 . . . . . . . . . . add -20-2s
.. 4s = f = 40 . . . . . . . The father's current age is 40.