How many litres of water should a milkman add to a 40-litre mixture of milk and water, which contains milk and water in the ratio of 6 : 4, such that the resultant mixture contains 60% water?

Using the rule of alligation, the initial mixture has 40% water and pure water has 100% concentration, while the target mixture has 60% water. The required ratio of initial mixture to pure water is (100 - 60) : (60 - 40), which simplifies to 40 : 20 or 2:1. Since the initial mixture is 40 litres, the milkman must add half that amount, which is 20 litres of water.

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