### Instruction: 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 Choices: (A) 27/7 sec (B) 20/7 sec (C) 33/7 sec (D) 21/7 sec (E) 32/7 sec.### Response: B
20/7 sec
They are moving in opposite directions, relative speed is equal to the sum of their speeds.
Relative speed = (54 + 72)*5/18 = 7*5 = 35 mps.
The time required = d/s = 100/35 = 20/7 sec.