A person standing on a railway platform noticed that a train took 21 seconds to completely pass through the platform which was 84m long and it took 9 seconds in passing him.Find the speed of the train? Select
A) 6m/sec
B) 5m/sec
C) 12m/sec
D) 7m/sec
E) None
Let the length of the train be x m.   then, then train covers x meters in 9 seconds and (x+84)m in 21 seconds.   Therefore, x/9=(x+84)/21 =63m   So, the length of the train=63m   Speed of the train=63/9= 
<gadget id="calculator">63/9</gadget>
<output>7</output>
7m/sec   Answer: Option 7m/sec
<result>D</result>
A person standing on a railway platform noticed that a train took 21 seconds to completely pass through the platform which was 84m long and it took 9 seconds in passing him.Find the speed of the train?