Find the number of solid spheres, each of diameter 6 cm,

We have,

Radius of the sphere, $R=\frac{6}{2}=3 \mathrm{~cm}$,

Radius of the cylinder, $r=\frac{4}{2}=2 \mathrm{~cm}$ and

Height of the cylinder, $h=45 \mathrm{~cm}$

Now,

The number of solid spheres $=\frac{\text { Volume of the Cylinder }}{\text { Volume of the sphere }}$

$=\frac{\pi r^{2} h}{\left(\frac{4}{3} \pi R^{3}\right)}$

$=\frac{3 r^{2} h}{4 R^{3}}$

$=\frac{3 \times 2 \times 2 \times 45}{4 \times 3 \times 3 \times 3}$

$=5$

So, the number of solid spheres so moulded is $5 .$