A 60-litre mixture of milk and water has milk and water in the ratio $7 : 3$. How much water must be added so that the ratio becomes $7 : 5$?

Explanation

1. 01

   Milk = $\frac{7}{10} \times 60 = 42$ litres; water = 18 litres.

2. 02

   Milk does not change, so in the new ratio 7 parts = 42 litres and 1 part = 6 litres.

3. 03

   New water = $5 \times 6 = 30$ litres, so add $30 - 18 = 12$ litres.