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1. A coolie standing on a railway platform observe that a train going in one direction takes 4 second to pass him. Another train of same length going in opposite direction takes 5 seconds to pass him. The time taken (in seconds) by two train to cross each other will be:

A. 35 seconds

B. 36.5 seconds

C. $\frac{40}{9}$ seconds

D. 36 seconds

Solution:

Let length of each train be X m, then

\begin{aligned} S_{1} = \frac{X}{4} and \\ S_{2} = \frac{X}{5} \\ Required time to cross each other, \\ = \frac{2 X}{\frac{X}{4} + \frac{X}{5}} \\ = \frac{40}{9} seconds \end{aligned}

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