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

Let the rate of 120 quintals be Rs.120.
For 25% gain = 120 $$\times \frac{125}{100} = Rs.150
20% of the 120 quintals = 20 $$\times \frac{120}{100}$$ = 24 quintals
remaining wheat = 120 - 24 = 96 quintals
20% of it was sold at 25% less
So, selling price of 20% wheat = 24 $$\times \frac{75}{100} = Rs.18 
Cost price for 96 quintals wheat = Rs.96
Selling price = 150 - 18 = 132
Gain = 132 - 96 = 36
Gain% = $$\frac{36}{96} \times 100 = 37\frac{1}{2}$$
$$\therefore$$ The correct answer is option C.