To gain 10\% on selling sample milk at the cost price of pure milk, the quantity of water to be mixed with 50 kg of pure milk is

Selling 50 kg milk mixed with x kg water at pure milk's CP means we effectively sell (50+x) kg at price of 50 kg. Water costs nothing, so profit comes from selling extra quantity. 10% profit means (50+x)/50 = 1.10, so x = 5 kg. Option A (2.5 kg) gives only 5% profit, while C (7.5 kg) and D (10 kg) would give 15% and 20% respectively.