50 litres of a mixture have milk and water in the ratio 7 : 3. How much water must be added to make the ratio 7 : 5?

Explanation

1. 01

   Milk is $\frac{7}{10} \times 50 = 35$ litres and water 15 litres. The milk stays 35.

2. 02

   For 7 : 5, water must be $\frac{5}{7} \times 35 = 25$ litres, so add $25 - 15 = 10$ litres.