Answer:
3 seconds.
Step-by-step explanation:
First calculate the time for each 20m length to pass through the bridge.
The train takes 3600 seconds to travel 144,000 m
So it takes 3600/144000 seconds to travel 1m.
and 3600 * 20 / 144000 seconds to travel 20 m.
As the train is 120 m long and there are 6 * 20 m in 120m the answer is:
3600 * 20 * 6 / 144000 seconds.
= 3 seconds.