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1. Two trains of lenths 120 m and 90 m are running with speed of 80 km/hr and 55 km/hr respectively towards each other on parallel lines. If they are 90 m apart, after how many seconds they will cross each other?

A. 5.6 sec

B. 7.2 sec

C. 8 sec

D. 9 sec

Solution:

\begin{aligned} Relative speed \\ =  (80 + 55)km/hr \\ =  135 km/hr \\ = (135 \times \frac{5}{18}) m / \sec \\ = (\frac{75}{2}) m / \sec \\ Distance covered \\ =  (120 + 90 + 90)m \\ =  300m \\ Required time \\ = (300 \times \frac{2}{75}) \sec \\ = 8 \sec \end{aligned}

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