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1. Two trains 140 m and 160 m long run at the speed of 60 km/hr and 40 km/hr respectively in opposite directions on parallel tracks. The time (in seconds) which they take to cross each other, is:

A. 9

B. 9.6

C. 10

D. 10.8

Solution:

\begin{aligned} Relative speed = (60 + 40) km/hr \\ = 100 \times \frac{5}{18} m/sec \\ = \frac{250}{9} m/sec . \\ Distance covered in crossing each other \\ = (140 + 160) m = 300 m \\ Required time \\ = 300 \times \frac{9}{250} sec \\ = \frac{54}{5} sec \\ = 10.8 sec \end{aligned}

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