A's age is the 5th part of his father's age.  After 15 years the age of A's father will be twice the age of B.  If B's 15th birthday is celebrated 25 years ago, then what is the present age of A?

13 years

B's present age = 15 + 25 = 40 years

A's father age 15 years after = 40 × 2 = 80 years

A's father present age = 80 - 15 = 65 years

A's age = 65 × \(\frac{1}{5}\) = 13 years