### Theory:

Sphere:
A sphere is a three-dimensional figure obtained by the revolution of a semicircle about its diameter as an axis.
Volume of a sphere:
Let $$r$$ be the radius of a sphere.

Volume of a sphere $$=$$ $\frac{4}{3}\mathrm{\pi }{r}^{3}$ cu. units

Demonstration of the volume of a sphere using right circular cones:
Let us take a sphere and two right circular cones of the same base radius and height.

Radius of a sphere $$=$$ $$r$$ units

Radius of two cones  $$=$$ $$r$$ units

Height of a sphere $$=$$ Diameter $$=$$ $$2r$$

Height of each cone $$=$$ Height of a sphere $$=$$ $$2r$$

Volume of a sphere $$=$$ Volume of $$2$$ cones

$$=$$ $2×\frac{1}{3}\mathrm{\pi }{r}^{2}h$

$$=$$ $2×\frac{1}{3}\mathrm{\pi }{r}^{2}×\left(2r\right)$ [Since $$h = 2r$$]

$$=$$ $\frac{4}{3}\mathrm{\pi }{r}^{3}$

Volume of a sphere $$=$$ $\frac{4}{3}\mathrm{\pi }{r}^{3}$ cu. units
Volume of a hollow sphere / spherical shell (volume of the material used):

Let $$r$$ be the inner radius and $$R$$ be the outer radius of the hollow sphere.

Volume of a hollow sphere $$=$$ Volume enclosed between the outer and inner spheres

$$=$$ $\frac{4}{3}\mathrm{\pi }{R}^{3}$ $$-$$ $\frac{4}{3}\mathrm{\pi }{r}^{3}$

$$=$$ $\frac{4}{3}\mathrm{\pi }\left({R}^{3}-{r}^{3}\right)$
Volume of a hollow sphere $$=$$ $\frac{4}{3}\mathrm{\pi }\left({R}^{3}-{r}^{3}\right)$ cu. units
Important!
The value of $$\pi$$ should be taken as $\frac{22}{7}$ unless its value is shared in the problem.