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?  Choose the most appropriate option:
A) 20 / 8 sec	B) 20 / 4 sec	C) 20 / 7 sec	D) 22 / 7 sec	E) 60 / 7 sec

Rationale: "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 . answer : c"
Answer: C