Discussion
Two trains, measuring 150 meters and 120 meters in length, are moving in opposite directions on parallel tracksone at a speed of 72 km/h and the other at 36 km/h. They will cross each other in :
Answer: C
The trains cross each other in 9 seconds.
Moving in opposite directions, their relative speed is the sum of their speeds, and to cross they must cover the sum of their lengths:
\[ \begin{aligned} v &= 72 + 36 = 108 \text{ km/h} \\ &= 108 \times \tfrac{5}{18} = 30 \text{ m/s} \\ t &= \frac{150 + 120}{30} = 9 \text{ s} \end{aligned} \]
If the trains moved in the same direction, the relative speed would be 36 km/h (10 m/s) and the time 27 seconds, which is not offered. 5, 7 and 11 seconds do not fit the given lengths and speeds.
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