A certain scheme of investment in simple interest declares that it triples the investment in 8 years. If you want to quadruple the money through that scheme for how many years you have to invest for = ?

$$\eqalign{ & {\text{P}} + \frac{{P \times {\text{r}} \times {\text{t}}}}{{100}} = 3{\text{P}} \cr & \Rightarrow 1 + \frac{{rt}}{{100}} = 3 \cr & \Rightarrow \frac{{{\text{rt}}}}{{100}} = 2 \cr & \Rightarrow {\text{r}} = \frac{{2 \times 100}}{8} = 25\% \cr & {\text{so, }}\,\left( {1 + \frac{{{\text{rt}}}}{{100}}} \right) = 4 \cr & \Rightarrow \frac{{{\text{rt}}}}{{100}} = 3 \cr & \Rightarrow {\text{t}} = \frac{{3 \times 100}}{{25}} \cr & \Rightarrow {\text{t}} = 12\,{\text{years}} \cr} $$