The diameter of a spare is 8 cm. It is melted and drawn into a wire of diameter 3 mm. The length of the wire is :

Answer: Option B

Let the length of the wire be h
Then,
$$\begin{array}{rl}& \pi \times \frac{3}{20}\times \frac{3}{20}\times h=\frac{4}{3}\pi \times 4\times 4\times 4\\ & \Rightarrow h=\left(\frac{4\times 4\times 4\times 4\times 20\times 20}{3\times 3\times 3}\right)cm\\ & \Rightarrow h=\left(\frac{102400}{27}\right)cm\\ & \Rightarrow h=3792.5cm\\ & \Rightarrow h=37.9m\end{array}$$