# The simple interest on a certain sum of money at the rate of 5% per annum for 8 years is Rs. 840. Rate of interest for which the same amount of interest can be received on the same sum after 5 years is ?

A. 7% B. 8% C. 9% D. 10% Answer: Option B
$\begin{array}{l}\text{ Case }-\mathbf{I}:\\ \mathrm{SI}\%=\mathrm{R}\%\times\mathrm{t}\\ \mathrm{SI}\%=5\%\times8\\ =40\%\\ \text{ Case }-\mathbf{II}:\\ \mathrm{SI}\%=5\times\mathrm{r}\%\\ \text{ According to the question }\\ \Rightarrow40\%=5\times\mathrm{r}\%\\ \Rightarrow\mathrm{r}\%=8\%\end{array}$