Mohan lent some amount of money at 9% simple interest and an equal amount of money at 10% simple interest each for two years. If his total interest was Rs. 760, what amount was lent in each case ?

Answer: Option D

Let the amount invested = Rs. P
According to the questions,
$$\begin{array}{rl}& \iff \frac{\text{P}\times 9\times \text{2}}{100}+\frac{\text{P}\times \text{10}\times \text{2}}{100}=760\\ & \iff \frac{18\text{P + 20P}}{100}=760\\ & \iff 38\text{P = 76000}\\ & \iff \text{P = 2000}\end{array}$$

Alternate
Total interest percent
$$\begin{array}{rl}& \text{ = }\left(9\times 2\right)\%+\left(10\times 2\right)\%\\ & \Rightarrow 38\%=760\\ & \Rightarrow 100\%=2000\end{array}$$
Hence required principal
= Rs. 2000