A 40-litre mixture is 25% water and the rest milk. How much milk must be added so that water makes up only 20% of the mixture?

Explanation

1. 01

   Water = $0.25 \times 40 = 10$ litres, and it does not change.

2. 02

   For 10 litres to be 20%, the total must be $\frac{10}{0.20} = 50$ litres.

3. 03

   Milk to add = $50 - 40 = 10$ litres.