Explanation:
the volume of a cylinder is
ground area × height
the ground area is a circle, and the area of a circle is
pi×r²
with r being the radius, which is always half of the diameter.
so, in our case
r = 5.5/2 = 2.75 cm
height = 20 cm
so, the volume of the cylinder is
pi×2.75² × 20 = pi×7.5625×20 = 475.1658889... cm³
the volume of a cube is
(side length)³
and the side length of a cube is therefore the cubic root of the volume.
now the volume of the cube has to be at least a large as the volume of the cylinder to be able to contain all the water from the cylinder.
so, the side length has to be at least the cubic root of that volume (475.1658889... cm³).
which is 7.803361957... cm.