let

m = the age of the man now

s = the age of his son now

A man is now 4 times as old as his son:

m = 4s

in 20 years he will only be twice as old:

m + 20 = 2(s + 20)

by solving the system of equations:

m = 4s

m + 20 = 2(s + 20)

we find:

m = 40 years

s = 10 years

click here to see the step by step solution of the system of equations:

the man is 40 years old now.

the son is 10 years old now.