Two trains 200m and 300m long run at the speed of 70kmph and 50kmph in opposite directions in parallel tracks. The time which they take to cross each other is?
Choose one.
A) 10 sec  B) 15 sec  C) 18 sec  D) 20 sec  E) 22 sec

Rationale: "relative speed = 70 + 50 = 120 kmph * 5 / 18 = 100 / 3 m / s distance covered in crossing each other = 200 + 300 = 500 m required time = 500 * 3 / 100 = 15 sec answer is b"
Answer: B