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. After how much time will the trains meet?
Answer: C
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 meet, only the 100 m gap between them has to close:
\[ t = \frac{100}{35} = \frac{20}{7} \text{ sec} \]
Their lengths play no part until they begin to pass each other.
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