If a sum doubles in 16 years, how much will it be in 8 years ?

Let Sum = Rs. x. Then, S.I. = Rs. x, Time = 16 years
$$\eqalign{ & \therefore {\text{Rate}} = \left( {\frac{{100 \times x}}{{x \times 16}}} \right)\% = {\frac{25}{4}}\% = {6\frac{1}{4}}\% \cr & {\text{Now, sum}} = {\text{Rs}}{\text{. }}x, \cr & {\text{Time}} = 8{\kern 1pt} {\text{years}} \cr & {\text{Rate}} = 6\frac{1}{4}\% \cr & \therefore {\text{S}}{\text{.I}}{\text{.}} = {\text{Rs}}{\text{.}}\left( {\frac{{x \times 25 \times 8}}{{100 \times 4}}} \right) \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{. }}\frac{x}{2} \cr & {\text{So,}} \cr & {\text{Amount}} = {\text{Rs}}{\text{.}}\left( {x + \frac{x}{2}} \right) \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{. }}\frac{{3x}}{2} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 1\frac{1}{2}{\text{ times}} \cr} $$