A rectangular water tank is $90cm$ wide and $40cm$ deep. If it can contain $576$ litres of water, what is its length?

Let the length of the rectangular tank be $l$ cm.

Given its width is $90$ cm, and depth is $40$ cm.

Now the volume of the rectangular tank is $90\times 40\times l=3600l$ cm${}^3$.

Given it can hold $576$ litres $=576,000$ cm${}^3$ of water.

Then

$3600l=576,000$

or,$l=\frac{5760}{36}$

or, $l=160$.

So length is $160$ cm.