From a container full of milk, 10 liters ($l$) was drawn and replaced by water. This process is repeated one more time. The ratio of quantity of milk and water left in the container is 4:5. Then the capacity of the container is:

$30l$

The correct answer is Option (3) → $30l$ **

Let the capacity of the container be $V$ litres.

Milk remaining after one replacement:

$\frac{V-10}{V}$

Milk remaining after second replacement:

$\left(\frac{V-10}{V}\right)^{2}$

Final milk : water = $4:5$

Total = 9 parts → milk = $\frac{4}{9}V$

So,

$V\left(\frac{V-10}{V}\right)^{2}=\frac{4}{9}V$

Cancel $V$:

$\left(\frac{V-10}{V}\right)^{2}=\frac{4}{9}$

Take square root:

$\frac{V-10}{V}=\frac{2}{3}$

$3(V-10)=2V$

$3V-30=2V$

$V=30$

Capacity = 30 litres