A dealer sells two blankets at ₹4200 each. On the first he gains 25%; on the two taken together he neither gains nor loses. His loss percent on the second blanket is:

The first blanket cost 4200/1.25 = ₹3360. Since the pair breaks even, the two together cost ₹8400, so the second cost ₹5040 and was sold at a loss of ₹840, i.e. 840/5040 = 16.67%. Losing 25% to cancel a 25% gain is wrong because the two costs are different; and 840/4200 = 20% works out the loss on the selling price instead of the cost price.