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

A. 10

B. 12

C. 15

D. 20

Solution:

\begin{aligned} Speed of the 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)m/sec \\ =  20 m/sec \\ ∴ Required time \\ = \frac{(120 + 120)}{20} \sec \\ = 12 \sec \end{aligned}

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