Question 1122430
D = Dad's age now
S= Son's age now

D = 4 S

Four years ago, Dad was D-4 years old and the son was S-4

(D-4)+(S-4) = 37

D+S-8 = 37

D+S=45

But we know D=4S so substitute 

4S+S = 45

S= 9  (and D= 36 in case you wonder)