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:

Option :

$\begin{array}{rl}& \text{Let}\phantom{\rule{thinmathspace}{0ex}}\text{the}\phantom{\rule{thinmathspace}{0ex}}\text{ivestment}\phantom{\rule{thinmathspace}{0ex}}\text{in}\phantom{\rule{thinmathspace}{0ex}}9\mathrm{%}\phantom{\rule{thinmathspace}{0ex}}\text{stock}\phantom{\rule{thinmathspace}{0ex}}\text{be}\phantom{\rule{thinmathspace}{0ex}}Rs.\phantom{\rule{thinmathspace}{0ex}}x.\\ & \text{Then,}\phantom{\rule{thinmathspace}{0ex}}\text{investment}\phantom{\rule{thinmathspace}{0ex}}\text{in}\phantom{\rule{thinmathspace}{0ex}}10\mathrm{%}\phantom{\rule{thinmathspace}{0ex}}\text{stock}\\ & =Rs.\phantom{\rule{thinmathspace}{0ex}}\left(9800-x\right).\\ & \frac{9}{75}×\phantom{\rule{thinmathspace}{0ex}}x\phantom{\rule{thinmathspace}{0ex}}=\frac{10}{80}\phantom{\rule{thinmathspace}{0ex}}×\phantom{\rule{thinmathspace}{0ex}}\left(9800-x\right)\\ & ⇒\frac{3x}{25}=\frac{9800-x}{8}\\ & ⇒24x=9800×25-25x\\ & ⇒49x=9800×25\\ & ⇒x=5000.\end{array}$