A vessel is in the shape of a square prism with length of base edge 20cm. It can contain 6l. of water. Find the depth of the vessel

A square is a three dimensional figure whose bases are squares and since length of one base is 20 cm, then length of other base will be also 20 cm.
Suppose depth of vessel is h cm.
So volume of vessel = $20 \mathrm{cm}\times 20 \mathrm{cm}\times h \mathrm{cm} = 400h {\mathrm{cm}}^3$
and the given volume of vessel = 6 l  = $\left(6\times 1000\right) {\mathrm{cm}}^3 = 6000 {\mathrm{cm}}^3$. so we have;
$$\begin{gathered} 400h = 6000 \\ \Rightarrow h = \frac{6000}{400} = 15 \end{gathered}$$
Therefore depth of vessel is 15 cm.