How much time will a train of length 400 m moving at a speed of 72 kmph take to cross another train of length 300 m, moving at 36 kmph in the same direction?  Answers.
A) 40 sec B) 50 sec C) 60 sec D) 70 sec E) 80 sec

Rationale: "the distance to be covered = sum of their lengths = 400 + 300 = 700 m . relative speed = 72 - 36 = 36 kmph = 36 * 5 / 18 = 10 mps . time required = d / s = 700 / 10 = 70 sec . answer : d"
Answer: D