Answer:
300 meters
Explanation:
54km/hr is equal to 15m/s (54/3.6 to get m/s)
The train took 40 seconds to cross the bridge while moving 15m/s.
Therefore it traveled 40*15 = 600 meters.
But we have to consider the length of the train is 300 meters, so the total length of the bridge will be 600-300 meters
= 300 meters