A mixture of 40 litres of milk and water contains 10% water. How much water should be added so that water becomes 20% of the new mixture?

5 litres of water should be added.

In 40 litres with 10% water, water = 4 litres and milk = 36 litres.

Let w litres of water be added; milk stays 36.

Water becomes 20%: (4 + w) / (40 + w) = 0.2.

4 + w = 8 + 0.2w, so 0.8w = 4.

w = 5 litres.