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1. A train moves past a telegraph post and a bridge 264 m long in 8 seconds and 20 seconds respectively. What is the speed of the train?

A. 69.5 km/hr

B. 70 km/hr

C. 79 km/hr

D. 79.2 km/hr

Solution:

\begin{aligned} Let the length of the train be x metres \\ and its speed by y m/sec \\ Then, \frac{x}{y} = 8 \Rightarrow x = 8 y \\ Now, \frac{x + 264}{20} = y \\ \Rightarrow 8 y + 264 = 20 y \\ \Rightarrow y = 22 \\ ∴ Speed = 22 m/sec \\ = 22 \times \frac{18}{5} km/hr \\ = 79.2 km/hr \end{aligned}

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