A sum of money placed at compound interest (compounding annually) doubles itself in 10 years. Find the time in which it will amount to 4 times itself.

Correct option is A

Given:
Amount doubles in 10 years.
Formula Used:
​$$\text {Amount} = \text{Principal} \times \left(1 + \frac{\text{Rate}}{100}\right)^{\text{Time}}$$​​
Solution:
P(1+(R/100))^10 = 2P
1+(R/100)^10 = 2
(1+(R/100))^T = 4 = 2^2
2 x 10 = 20yrs 

​Alternate Method:
For compound interest doubling: 2^n = Amount multiple

(1 + R/100)^10 = 2
Doubling time to quadruple: 2 × 10 = 20 years