Discussion
Two trains of length 100 m and 200 m are 100 m apart. They start moving towards each other on parallel tracks, at speeds 54 kmph and 72 kmph. In how much time will the trains cross each other?
Answer: B
Trains in the same direction close at the difference of their speeds; in opposite directions, at the sum.
\[ 1 \text{ kmph} = \frac{5}{18} \text{ m/s} \]
\[ \text{relative speed} = 54 + 72 = 126 \text{ kmph} = 35 \text{ m/s} \]
To cross completely, the gap and both lengths must be covered:
\[ 100 + 100 + 200 = 400 \text{ m} \]
\[ t = \frac{400}{35} = \frac{80}{7} \text{ sec} \]
Meeting takes only \( \tfrac{20}{7} \) seconds; crossing takes four times as long.
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