Question

A sum of money doubles itself in 5 years. In how many years will it become four fold (if interest is compounded)?

• A. 15
• B. 10
• C. 20
• D. 12

Answer 1

The correct option is B 10

Let sum = x

Then, $2x=x{[1+\frac{R}{100}]}^5$

$\Rightarrow [1+\frac{R}{100}]=2^{1/5}$ ... (i)

$4x=x{[1+\frac{R}{100}]}^T$

$\Rightarrow 4=[2^{1/5}{]}^T$

$\Rightarrow 2^2=2^{T/5}$

$\Rightarrow \frac{T}{5}=2$

$\therefore T=10yrs.$