A tank holds 100 litres of milk. 10 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 replacement keeps $1 - \frac{10}{100} = \frac{9}{10}$ of the milk.

2. 02

   After 3 times: $100\left(\frac{9}{10}\right)^{3} = \frac{729}{10}$ litres.