Two trains of length 100 meters and 200 meters are 840 meters apart. They are moving towards each other on parallel tracks, at speeds of 54 km/h and 72 km/h. After how many seconds will the trains meet?
Pick.
A) 20  B) 22  C) 24  D) 26  E) 28

Rationale: "the speeds are 54000 / 3600 = 15 m / s and 72000 / 3600 = 20 m / s the relative speed is 35 m / s . time = 840 / 35 = 24 seconds the answer is c ."
Answer: C