A tank holds 64 litres of pure milk. 16 litres are taken out and replaced with water. This is done once more. How much milk is left in the tank?

Explanation

1. 01

   Each operation leaves $\frac{64 - 16}{64} = \frac{3}{4}$ of the milk.

2. 02

   After two operations: $64 \times \left(\frac{3}{4}\right)^2 = 64 \times \frac{9}{16} = 36$ litres.