A sum of money place at compound interest doubles itself in 4 years. In how many years will it amount to eight times itself?

A. 72
B. 45
C. 37
D. 79
Explanation:
Sum of the marks for the 12 students = 12 * 36 = 432. The marks of each student are doubled, the sum also will be doubled.
The new sum = 432 * 2 = 864. So, the new average = 864/12 = 72