A vessel contains 100 litres of milk. From this, 10 litres of milk was taken out and replaced by water. The process was repeated further two times. How much milk is now contained in the vessel ?

Remaining quantity of milk in the vessel

$=$ Initial quantity $\left(1-\frac{\text { quantity taken out }}{\text { Initial quantity }}\right)^{n}$

$=100\left(1-\frac{10}{100}\right)^{3}=100\left(1-\frac{1}{10}\right)^{3}$

$=100 \times \frac{9}{10} \times \frac{9}{10} \times \frac{9}{10}=72.9 \text { litres }$