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1. A train 800 metres long is running at a speed of 78 km/hr. If it crosses a tunnel in 1 minute, then the length of the tunnel (in meters) is:

A. 130

B. 360

C. 500

D. 540

Solution:

\begin{aligned} Speed = (78 \times \frac{5}{18}) m/sec \\ = \frac{65}{3} m/sec \\ Time = 1 minute = 60 second . \\ Let the length of the tunnel be x metres . \\ Then, \frac{800 + x}{60} = \frac{65}{3} \\ \Rightarrow 3 (800 + x) = 3900 \\ \Rightarrow x = 500 \end{aligned}

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