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1. Two trains of equal lengths take 10 seconds and 15 seconds respectively to cross a telegraph post. If the length of each train be 120 metres, in what time (in seconds) will they cross each other travelling in opposite direction?

A. 10

B. 12

C. 15

D. 20

Solution:

\begin{aligned} Speed of the first train \\ = \frac{120}{10} m/sec \\ = 12 m/sec \\ Speed of the second train \\ \frac{120}{15} m/sec \\ = 8 m/sec \\ Relative speed = 12 + 8 = 20 m/sec \\ ∴ Required time \\ = \frac{120 + 120}{20} sec \\ = 12 sec \end{aligned}

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