If the sum doubles (x2) in 15 years, then it to become 2*2*2 = 8 times as large it should double three times, so three 15 years time period are needed, which is 3*15 = 45 years.
This justifies what answer for what question? Q & A:
A sum of money doubles itself at a compound interest in 15 years . In how may years it will become 8 times ??
Options:
(A) 30 years
(B) 40 years
(C) 45 years
(D) 50 years
(E) 60 years
(C)