The radius of the wheel of a vehicle is 70 cm. The wheel makes 10 revolutions in 5 seconds. The speed of the vehicle is :

Answer: Option B

Distance covered in 5 sec :
$$\begin{array}{rl}& =\left(2\times \frac{22}{7}\times 70\times 10\right)cm\\ & =4400cm\\ & =44m\end{array}$$
Distance covered in 1 sec :
$$\begin{array}{rl}& =\left(\frac{44}{5}\right)m\\ & =8.8m\end{array}$$
∴ Speed :
$$\begin{array}{rl}& =8.8m/\sec \\ & =\left(8.8\times \frac{18}{5}\right)km/hr\\ & =31.68km/hr\end{array}$$