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

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