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1. Two trains A and B start running together from the same point in the same direction, at the speed of 60 kmph and 72 kmph respectively. If the length of each of the trains is 240 meters, how long will it take for B to cross train A?

A. 1 min 12 sec

B. 1 min 24 sec

C. 2 min 12 sec

D. 2 min 24 sec

Solution:

\begin{aligned} Relative speed \\ =  (72 - 60) km/hr \\ =  12 km/hr \\ = (12 \times \frac{5}{18}) m / \sec \\ = (\frac{10}{3}) m / \sec \\ Total distance covered \\ =  Sum of lengths of trains \\ =  (240 + 240) m \\ = 480 m \\ Time taken \\ = (480 \times \frac{3}{10}) \sec \\ = 144 \sec \\ = 2 min 24 s e c \end{aligned}

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