In 90 litres of a mixture of milk and water, the ratio of milk and water is 4 ∶ 1. How much water should be added in the mixture so that the ratio of milk to water becomes 3 ∶ 1?

In the initial 90 litres of mixture, the milk and water ratio of 4 : 1 means there are 72 litres of milk and 18 litres of water. Let x be the amount of water to add to achieve a 3 : 1 ratio, so the equation becomes 72 / (18 + x) = 3 / 1. Solving for x gives 72 = 54 + 3x, which simplifies to 3x = 18 and x = 6 litres.

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