Question 327365
Let the present age of bob be x years.
Let the present age of bob's father be y.
Therefore we get, y = 3x.( using the condition "bob's father is 3 times as old as bob).
This gives 3x - y = 0 ..........(1)
Four years ago, 

The age of bob = x - 4.
The age of bob's father = y - 4.

Therefore, we get , y - 4 = 4 ( x-4) (using the condition that four years ago he was four times older)
This gives y-4 = 4x - 16
4x - y = 12.........(2)

Solving the linear equations (1) and (2) we get
x = 12   and y=36.


Sol:

Bob's present age x = 12 years
Father's present age y = 36 years