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1. If the wheel of the engine of a train $4 \frac{2}{7}$ metres in circumference makes 7 revolutions in 4 seconds, then the speed (in km/hr) of the train is :

A. 27

B. 28

C. 29

D. 30

Solution:

Distance covered in 4 sec :

\begin{aligned} = (\frac{30}{7} \times 7) m \\ = 30 m \end{aligned}

Distance covered in 1 sec =

\frac{30}{4}

Distance covered in 1 revolution :

\begin{aligned} = (\frac{30}{4}) m \\ = \frac{15}{2} m \end{aligned}

∴ Required speed :

\begin{aligned} = (\frac{15}{2}) m / s \\ = (\frac{15}{2} \times \frac{18}{5}) k m / h r \\ = 27 k m / h r \end{aligned}

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