A sum becomes its double in 10 years. Find the annual rate of simple interest.

Answer: Option C

  Let Sum be Rs. x and Let S.I. = Rs. x
$$\begin{array}{rl}& \text{Time}=10\text{years}\\ & \therefore \text{Rate}=\frac{\text{S}\text{.I}\text{.}\times \text{100}}{\text{Principal}\times \text{Time}}\\ & =\frac{x\times 100}{x\times 10}\\ & =10\%\text{per}\text{annum}\text{.}\end{array}$$