In what ratio must water be mixed with milk costing ₹60 per litre so that, selling the mixture at ₹60 per litre, the shopkeeper makes a profit of 25%?

A 25% profit at a selling price of ₹60 means the mixture must cost him 60/1.25 = ₹48 per litre. Water costs nothing, so by alligation water : milk = (60 − 48) : (48 − 0) = 12 : 48 = 1 : 4. (Check: 1 litre of water with 4 litres of milk costs ₹240 for 5 litres, which sells for ₹300 — a profit of ₹60 on ₹240 = 25%. Reading the 1 : 5 off the total instead of the milk is the usual slip.)