A sum doubles itself in 5 years at compound interest, compounded annually. In how many years will it become 8 times itself?

Explanation

1. 01

   At compound interest, the sum doubles every 5 years.

2. 02

   $8 = 2 \times 2 \times 2$, so it must double three times.

3. 03

   Time = $3 \times 5 = 15$ years.