Question

a train crosses a platform 90m long in 30 s and a man standing on the platform in 15 s . find the speed of the train

Answer 1

speed=distance/time
$x = 90 \div 0.5 \\ x = 180$
therefore speed=180m/s
=648km/h

Answer 2

In 30 sec train complete the distance = Train lenghth + 90 meter plateform
But when the train crossed a person who is standing on the plateform it takes just half time bcoz the train complete its own lenghth to crossed that person so train lenghth is also 90 meters
90 mtr in 15 sec = 6mtr/sec
In kilometer its speed is 21.6 kmph