Question 12424
Let x = son's present age
   7x = father's age
        

E:   7x + 5 = 4(x + 5)
     7x + 5 = 4x + 20
     7x -4x = 20 - 5
         3x = 15
          x = 15/3
          x = 5

A:      x = son's age
 7x = 7(5)= 35 - father's age

Checking: 7(5) + 5 = 4(5 + 5)
            35 + 5 = 20 + 20
                40 = 40