A 50-litre mixture has milk and water in the ratio $3 : 2$. By mistake, 10 litres of water are added. How much milk must now be added to bring the ratio back to $3 : 2$?

Explanation

1. 01

   At first: milk 30 litres, water 20 litres. After the mistake: water 30 litres.

2. 02

   For $3 : 2$, milk must be $\frac{3}{2} \times 30 = 45$ litres.

3. 03

   Milk to add = $45 - 30 = 15$ litres.