A grain trader sells at cost price, but on every second sale he weighs with a stamped '1 kg' block that really weighs 900 g; on the sales in between he weighs honestly. Over a long run of such sales his gain percent is:

Take a pair of sales and a cost of ₹100 per kg. He hands over 1000 + 900 = 1900 g, which cost him ₹190, and collects the price of 2 kg = ₹200. Gain = 10 on 190 = 5.26%. (On the dishonest sale alone the gain is 100/900 = 11.11%, and halving that to 5.56% is close but wrong — the honest sale contributes no profit yet still adds a full kilogram of COST to the base. 10% is only the shortfall in the weight, measured on the wrong side.)