A container contains 40 liters of milk, From this container, 4 liters of milk was taken out and replaced by water. This process was further repeated two more times. How much milk is now there in the container?

Solution: Suppose a container contains  x units of  a liquid from which y units are taken out and replaced by a  water, after n operations quantity of  pure liquid.

$\begin{array}{l}=x{[1-\frac{y}{x}]}^{n}n=3\\ \Rightarrow 40\times \frac{9}{10}\times \frac{9}{10}\times \frac{9}{10}=\frac{729\times 4}{100}=29.16\end{array}$