A 60-litre drink is made of syrup, juice and water in the ratio 3 : 4 : 5. How many litres of water must be added so that syrup and water are in the ratio 1 : 3?

The 12 parts of 60 litres are 5 litres each, so the drink holds 15 L of syrup, 20 L of juice and 25 L of water. The syrup stays at 15 L, so syrup : water = 1 : 3 needs 45 L of water — that is 45 − 25 = 20 L more.