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1. Two trains of equal length, running in opposite directions, pass a pole in 18 and 12 seconds. The trains will cross each other in:

A. 14.4 seconds

B. 15.5 seconds

C. 18.8 seconds

D. 20.2 seconds

Solution:

Let length of each train be x meter.
Then, speed of 1^st train =

\frac{x}{18}

m/sec
Speed of 2^nd train =

\frac{x}{12}

m/sec
Now,
When both trains cross each other, time taken

\begin{aligned} = \frac{2 x}{\frac{x}{18} + \frac{x}{12}} \\ = \frac{2 x}{\frac{2 x + 3 x}{36}} \\ = \frac{2 x \times 36}{5 x} \\ = \frac{72}{5} \\ = 14.4 seconds \end{aligned}

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