In how many years a sum placed on compound interest becomes 8 times of itself, if it becomes twice in 6 years?

18

Assume the sum to be P.

Compounding annually,

Amount becomes double in 6 years,

A = P(1+ r/100)^T

2P = P(1+ r/100)^6

(1+r/100)^6 = 2

1+ r/100 = 2^(1/6) = 1.1224

r = 12.24

For amount to be 8 Times,

8P = P(1+r/100)^T

(1+12.24/100)^T = 8

(1.1224)^T = 8

T = 18

The correct answer is Option (2) → 18