A shopkeeper bought 120 quintals of wheat. 20% of it was sold at 25% loss. At what percent gain should he sell the rest to gain 25% on the whole transaction?

Answer: Option B

Quantity of wheat bought by shopkeeper = 120
Let the price of the 120 quintals of wheat = Rs. 120
According to the question
$$\begin{array}{rl}& 120\times \frac{1}{5}\times \frac{75}{100}+120\times \frac{4}{5}\times \frac{100+x}{100}=120\times \frac{125}{100}\\ & \Rightarrow 18+\frac{24}{25}\times \left(100+x\right)=150\\ & \Rightarrow \frac{24}{25}\times \left(100+x\right)=150-18\\ & \Rightarrow \frac{24}{25}\times \left(100+x\right)=132\\ & \Rightarrow 100+x=132\times \frac{25}{24}\\ & \Rightarrow 100+x=137.5\\ & \Rightarrow x=137.5-100\\ & \Rightarrow x=37.5\%\text{ or }37\frac{1}{2}\%\end{array}$$