A sum of money lent out at simple interest amounts to Rs. 720 after 2 years and to Rs.1020 after a further period of 5 years. The sum is

$$\eqalign{ & {\text{S}}{\text{.I}}{\text{. for 5 years}} \cr & = {\text{Rs}}{\text{.}}\left( {1020 - 720} \right) \cr & = {\text{Rs}}{\text{. 300}} \cr & {\text{S}}{\text{.I}}{\text{. for 2 years}} \cr & = {\text{Rs}}{\text{.}}\left( {\frac{{300}}{5} \times 2} \right) \cr & = {\text{Rs}}{\text{. }}120 \cr & \therefore {\text{Principal}} \cr & = {\text{Rs}}{\text{.}}\left( {{\text{720}} - 120} \right) \cr & = {\text{Rs}}{\text{. }}600 \cr} $$