Question 1002726
Call the father's and son's ages NOW, f and s.
f + 5 = 3(s + 5) = 3s + 15
f - 5 = 7(s - 5) = 7s - 35
Subtract equations and get
10 = -4s + 50
-40 = -4s 
s = 10
f = 40