If a principal amount doubles in value after 4 years at compound interest, how many years will it take for the amount to become eight times the original?

Explanation

Since the sum doubles every 4 years, it grows as follows: after 4 years it is 2 times the original, after 8 years it is 4 times, and after 12 years it becomes 8 times the initial amount. Therefore, it takes 12 years for the amount to become eight times the principal.