If the wheel of the engine of a train 42/7 metres in circumference makes 7 revolutions in 4 seconds, then the speed (in km/hr) of the train is :

Answer: Option A

Distance covered in 4 sec :
$$\begin{array}{rl}& =\left(\frac{30}{7}\times 7\right)m\\ & =30m\end{array}$$
Distance covered in 1 sec = $$\frac{30}{4}$$
Distance covered in 1 revolution :
$$\begin{array}{rl}& =\left(\frac{30}{4}\right)m\\ & =\frac{15}{2}m\end{array}$$
∴ Required speed :
$$\begin{array}{rl}& =\left(\frac{15}{2}\right)m/s\\ & =\left(\frac{15}{2}\times \frac{18}{5}\right)km/hr\\ & =27km/hr\end{array}$$