Rs. 9800 are invested partly in 9% stock at 75 and 10% stock at 80 to have equal amount of incomes. The investment in 9% stock is:

$$\eqalign{ & {\text{Let}}\,{\text{the}}\,{\text{ivestment}}\,{\text{in}}\,9\% \,{\text{stock}}\,{\text{be}}\,Rs.\,x \cr & {\text{Then,}}\,{\text{investment}}\,{\text{in}}\,10\% \,{\text{stock}} \cr & = Rs.\,\left( {9800 - x} \right) \cr & \frac{9}{{75}} \times \,x\, = \frac{{10}}{{80}}\, \times \,\left( {9800 - x} \right) \cr & \Rightarrow \frac{{3x}}{{25}} = \frac{{9800 - x}}{8} \cr & \Rightarrow 24x = 9800 \times 25 - 25x \cr & \Rightarrow 49x = 9800 \times 25 \cr & \Rightarrow x = 5000 \cr} $$