A sum of money at simple interest doubles in 7 years. It will become four times in:

Answer: Option B

$$\begin{array}{rl}& \text{Let sum}=\text{Rs}\text{. }x\\ & \text{Then,}\\ & \text{S}\text{.I}\text{.}=\text{Rs}\text{.}x\\ & \therefore \text{Rate}\%\\ & =\left(\frac{100\times x}{x\times 7}\right)\%\\ & =\frac{100}{7}\%\\ & \text{Now, sum}=\text{Rs}\text{. }x\\ & \text{S}\text{.I}.=\text{Rs}\text{. }3x\\ & \text{Rate}=\frac{100}{7}\%\\ & \therefore \text{Total Time}\\ & =\left(\frac{100\times 3x}{x\times \frac{100}{7}}\right)\text{years}\\ & =21\text{years}\end{array}$$