A100-meter-long christmas train needs 30 seconds to cross a 400-meter-long bridge. assuming the train goes at a steady speed, how fast is it?

Length of train : 100
length of bridge : 400
distance covered by the train to cross the bridge : (100 + 400) = 500
time taken by the train to cross : 30

speed = distance / time
speed = 500/30
speed = 16.67 meters per second <===