Abridge is 500m long. a 100m long train crosses it at a speed of 30m/s. find the time taken by the train to cross it.

For the nose of the train to cross from one side of the bridge
to the other takes 500m.  But then the nose must travel another
100m to get every bit of the train off of the bridge.  So from the time
the nose enters onto the bridge, it must travel another 600m. 

That will take                     (600 m) / (30 m/sec) = 20 seconds.