A can holds 40 litres of milk. 4 litres are taken out and replaced with water, and this is done three times in all. How much milk is left?

Explanation

1. 01

   Each time, $\frac{4}{40} = \frac{1}{10}$ of what is in the can goes, so $\frac{9}{10}$ of the milk stays.

2. 02

   After three times: $40 \times \left(\frac{9}{10}\right)^{3} = 40 \times 0.729 = 29.16$ litres.