A person having bought goods for Rs. 400 sells half of it at a gain of 5%, at what gain % must he sell the remainder so as to gain 20% on the whole?

Answer: Option D

To gain 20% on whole he must sell all good for,
Rs. 400 + 20% of 400 = 480
As he get 5% gain on half of the goods i.e.
200 + 5% of 200 = 210
So required balance = 480 - 210 = 270
He must gain Rs. 70 on rest Rs. 200
% gain on remainder goods = $$\frac{70\times 100}{200}$$   = 35%