From a container filled with milk, $9 \; \text{litres}$ of milk are drawn and replaced with water. Next, from the same container, $9 \; \text{litres}$ are drawn and again replaced with water. If the volumes of milk and water in the container are now in the ratio of $16:9,$ then the capacity of the container, in $\text{litres},$ is

$\textbf{PS:}$ Suppose, a container contains $`x\text{’}$ units of a liquid from which $`y\text{’}$ units are taken out and replaced by water. After $`n\text{’}$ operations quantity of pure is $x\left(1-\frac{y}{x}\right)^{n}.$