$\displaystyle 1000$ small thermocol balls of radius $\displaystyle 0$•$\displaystyle 5$ cm are kept in a spherical balloon of radius $\displaystyle 20$ cm . Find the volume of air in the balloon.

Volume of air in the ballon = Volume of spherical balloon - Volume of $\displaystyle 1000$ thermocol balls \[\begin{aligned} & =\frac{4}{3} \times \frac{22}{7} \times(20)^{3}-1000 \times \frac{4}{3} \times \frac{22}{7} \times(0.5)^{3} \\ & =33000 \mathrm{~cm}^{3} \end{aligned} \]