[15 years] A sum of money placed at C.I doubles itself in 5 years. It will amount to eight times itself in :

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Aptitude

TNPSC G4

10th CBSE Maths

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

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