A sum of money at simple interest amounts to Rs. 815 in 3 years and to Rs. 854 in 4 years. The sum is

$$\eqalign{ & {\text{S}}{\text{.I}}{\text{. for 1 year}} \cr & = {\text{Rs}}.\left( {854 - 815} \right) \cr & = {\text{Rs}}.39 \cr & {\text{S}}{\text{.I}}{\text{. for 3 years}} \cr & = {\text{Rs}}{\text{.}}\left( {39 \times 3} \right) \cr & = {\text{Rs}}{\text{. }}117 \cr & \therefore \text{Principal} \cr & = {\text{Rs}}{\text{.}}\left( {854 - 117} \right) \cr & = {\text{Rs}}{\text{. }}698 \cr} $$