A father tells his son, "I was of your present age when you were born". If the father is 36 now, how old was the boy five years back ?

Explanation:-

Let the father's age be x and the son's age be y.
Then, x - y = y or x = 2y,
Now, x = 36. So, 2y = 36 or y = 18.
Therefore Son's present age = 18 years.
So, son's age 5 years ago = 13 years.