Fifty litres of a mixture of milk and water contains 30 percent of water. This mixture is added to eighty litres of another mixture of milk and water that contains 20 percent of water. Then how many litres of water should be added to the resulting mixture to obtain a final mixture that contains 25 percent of water?

The first mixture contains 15 litres of water and the second contains 16 litres, giving a total of 31 litres of water in the combined 130 litres. Let x be the litres of water added to achieve a 25 percent concentration, so 31 plus x equals 0.25 times (130 plus x). Solving this equation yields x equals 2. The required amount of water to add is 2 litres.

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