A 60-litre mixture has milk and water in the ratio $4 : 1$. How many litres of the mixture must be drawn off and replaced with water so that milk and water are in the ratio $1 : 1$?

Explanation

1. 01

   Milk now = 48 litres. For $1 : 1$ in 60 litres, milk must fall to 30 litres.

2. 02

   Drawing $x$ litres removes the fraction $\frac{x}{60}$ of the milk: $48\left(1 - \frac{x}{60}\right) = 30$.

3. 03

   $1 - \frac{x}{60} = \frac{5}{8}$, so $x = 60 \times \frac{3}{8} = 22.5$ litres.