The time it takes for the sound to travel from the train to the observer on the bridge is given by:
distance = speed × time
The distance is the same as the distance the train travels during the duration of the horn signal, which is:
distance = speed × time = 81.0 m/s × 7.20 s = 583.2 m
The time it takes for the sound to travel this distance is:
time = distance / speed of sound = 583.2 m / 334.0 m/s = 1.744 s
Therefore, the horn signal lasts for 1.744 seconds for the observer on the bridge.