If a sum of money is placed at compound interest doubles itself in 5 years, then the same amount of money will be 8 times of itself in 25 years.

Let the sum be Rs. x. 

Then x becomes 2x in 5 years, 

2x becomes 4x in next 5 years, 

4x becomes 8x in next 5 years.
Hence, x becomes 8x in 15 years.