A sum of money placed at C.I doubles itself in 5 years. It will amount to eight times itself in :
15 years
20 years
12 years
10 years
Explanation:
Let the principal P and rate be r% . Then, 2P = P (1+r100)5(1+r100)5 or
(1+r100)5(1+r100)5 = 2
Let it be 8 times in t years . Then, 8p = p (1+r100)t(1+r100)t
or (1+r100)t(1+r100)t=8=(2)3(2)3 = ((1+r100)5)3=(1+r100)15((1+r100)5)3=(1+r100)15
t = 15 years
Let the principal P and rate be r% . Then, 2P = P (1+r100)5(1+r100)5 or
(1+r100)5(1+r100)5 = 2
Let it be 8 times in t years . Then, 8p = p (1+r100)t(1+r100)t
or (1+r100)t(1+r100)t=8=(2)3(2)3 = ((1+r100)5)3=(1+r100)15((1+r100)5)3=(1+r100)15
t = 15 years