a sum of rs 12000 deposited at compound interest becomes double after..., Compound Interest, General Mathematics

Question:A sum of Rs.`12000` deposited at compound interest becomes double after `5` years. After `20` years it will become :

A `Rs. 120000` 
B `Rs. 192000` 
C `Rs. 124000` 
D `Rs. 96000`

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+ AnswerB

+ Explanation`12000 xx (1 + R/100)^5`

`= 24000 ⇒(1 + R/100)^5 = 2`

`:. [(1 + R/100)^5]^4`

`= 2^4 = 16`

`or (1 + R/100)^20 = 16`

`or P(1 + R/100)^20 = 16p`

`or 12000(1 + R/100)^20`

`= 16 xx 12000`

`= 192000`.

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