A sum of Rs. 12,800 is invested separately at 15% per annum and the remaining at 12% per annum simple interest. If the total interest at the end of 3 years is Rs. 5,085, then how much money was invested at 15% per annum?

$$\eqalign{ & {\text{Sum}} = 12,800 \cr & {{\text{I}}^{{\text{st}}}}{\text{ part}} = x \cr & {\text{I}}{{\text{I}}^{{\text{nd}}}}{\text{ part}} = \left( {12,800 - x} \right) \cr & x \times \frac{{15}}{{100}} + \left( {12,800 - x} \right)\frac{{12}}{{100}} = \frac{{5,085}}{3} \cr & \frac{{3x}}{{100}} + 128 \times 12 = 1,695 \cr & \frac{{3x}}{{100}} = 1,695 - 1,536 \cr & \frac{{3x}}{{100}} = 159 \cr & {\text{then }}x = 5,300 \cr} $$