A 70-litre mixture of milk and water contains 10% water. How many litres of water need to be added to increase the water content to 12.5%?

Explanation

The amount of milk in the original mixture is 90% of 70 litres, which is 63 litres. After adding water, milk will make up 87.5% of the new mixture. Let the total volume after adding water be (70 + x) litres. Then, 87.5% of (70 + x) equals 63 litres: (87.5/100) * (70 + x) = 63. Solving for x gives x = 2 litres. Therefore, 2 litres of water must be added.