Question:A sum of money placed at compound interest doudles itself in `5` years. It will amount to eight times itself in : 

A `10 years` 
B `12 years` 
C `15 years` 
D `20 years` 

+ Answer
C 

+ Explanation
`p (1 + R/100)^5`

`= 2p ⇒ (1 + R/100)^5 = 2`

Let `p (1 + R/100)^n`

`= 8p.` Then,

`(1 + R/100)^n = 8 = 2^3`

`= {(1 + R/100)^5}^3`

`= (1 + R/100)^15`

`:. n = 15 years`.

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Irin Moni

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