A vessel contains 40 litres of pure milk. Each time, 4 litres are drawn out and replaced with water. How much milk remains after two such operations?

Explanation

1. 01

   Each operation keeps $\frac{40 - 4}{40} = \frac{9}{10}$ of the milk.

2. 02

   After two operations: $40 \times \left(\frac{9}{10}\right)^2 = 40 \times 0.81 = 32.4$ litres.