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

∵ Milk = 90% 0f 40 = 36 litres and water = 4 litres.
Let L litre water to be added in order to make 20% water in new mixture.
⇒ {(4 + L) / (40 + L)} x 100 = 20
⇒ 20 x (40 + L) = 100 x (4 + L)
⇒ 80 x L = 400
∴ L = 5 litres .