If the simple interest for 6 years be equal to 30% of the principal, it will be equal to the principal after

$$\eqalign{ & {\text{Let sum}} = {\text{Rs}}{\text{. }}x \cr & {\text{Then,}} \cr & {\text{S}}{\text{.I}}{\text{.}} = 30\% \,{\text{of}}\,{\text{Rs}}{\text{.}}\,x \cr & \,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{.}}\frac{{3x}}{{10}} \cr & {\text{Time}} = 6\,{\text{years}}{\text{.}} \cr & \therefore {\text{Rate}} = \left( {\frac{{100 \times 3x}}{{10 \times x \times 6}}} \right)\% \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 5\% \cr & {\text{Now, sum}} = {\text{Rs}}{\text{. }}x \cr & {\text{S}}{\text{.I}}{\text{.}} = {\text{Rs}}{\text{. }}x \cr & {\text{Rate}} = 5\% \cr & \therefore {\text{Time}} = \left( {\frac{{100 \times x}}{{x \times 5}}} \right){\text{years}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 20\,{\text{years}} \cr} $$