If simple interest on Rs. 600 for 4 years and on Rs. 600 for 2 years combined together is Rs.180, find the rate of interest.

$$\eqalign{ & {\text{Let the rate of be R}}\% {\text{p}}{\text{.a}}{\text{.}} \cr & Then, \cr & {\frac{{600 \times R \times 4}}{{100}}} + {\frac{{600 \times R \times 2}}{{100}}} = 180 \cr & \Rightarrow 2400{\text{R}} + 1200{\text{R}} = 18000 \cr & \Rightarrow 3600{\text{R}} = 18000 \cr & \Rightarrow {\text{R}} = 5\% \cr} $$