Two trains of length 100 m and 200 m are 100 m apart. They start moving towards each other on parallel tracks, at speeds 90 kmph and 72 kmph. After how much time will the trains meet?  Answers
A) 20/8 sec B) 20/9 sec C) 20/7 sec D) 22/7 sec E) 60/7 sec
They are moving in opposite directions, relative speed is equal to the sum of their speeds.   Relative speed = (90 + 72)*5/18 = 9*5= 
<gadget id="calculator">9*5</gadget>
<output>45</output>
45 mps.   The time required= d/s = 100/45 = 20/9 sec.Answer:20/9 sec
<result>B</result>
Two trains of length 100 m and 200 m are 100 m apart. They start moving towards each other on parallel tracks, at speeds 90 kmph and 72 kmph. After how much time will the trains meet?