Question 834802
A train moves with a constant velocity v = 20 m/s while passing through a 200-m long tunnel.
 If 15 s elapse between the moment the front of the train enters the tunnel, and when the back of the train leaves 
:
Using the front of the train as ref. It travels 200m, to the end of the tunnel
then has to travel the length (t) of the train,  for the train to clear the tunnel
the total dist traveled then = (t+200)
:
dist/speed = time
{{{((t+200))/20}}} = 15
t + 200 = 20(15)
t + 200 = 300
t = 300 - 200
t = 100 meters is the length of the train
:
:
That's a pretty fast train
{{{(20*3600)/1000}}} = 72 km/h