#### Question

Some plastic balls of radius 1 cm were melted and cast into a tube. The thickness, length and outer radius of the tube were 2 cm , 90 cm and 30 cm respectively. How many balls were melted to make the tube?

#### Solution

Radius of each plastic ball, *R* = 1 cm

Outer radius of the tube, *r*_{2} = 30 cm

Thickness of the tube = 2 cm

∴ Inner radius of the tube, *r*_{1} = Outer radius of the tube − Thickness of the tube = 30 − 2 = 28 cm

Length of the tube, *h* = 90 cm

Let the number of plastic balls melted to make the tube be *n*.

It given that plastic balls are melted to form the tube.

∴ *n* × Volume of each plastic ball = Volume of the tube

\[\Rightarrow n = \frac{\text{ Volume of the tube} }{\text{ Volume of each plastic ball } }\]

\[ \Rightarrow n = \frac{\pi\left( r_2^2 - r_1^2 \right)h}{\frac{4}{3}\pi R^3}\]

\[ \Rightarrow n = \frac{\left( {30}^2 - {28}^2 \right) \times 90}{\frac{4}{3} \times 1^3}\]

\[ \Rightarrow n = \frac{116 \times 3 \times 90}{4} = 7830\]

Thus, the number of plastic balls melted to make the tube are 7830.