.
In one minute, the front of the train moves forward from the one end of the platform by 15 m/s * 60 seconds = 900 meters.
Since the problem says that the train passes (passes -- not crosses (!) ) the platform in 1 minute, it means that
the rear of the train comes alongside with the other end of the platform.
Therefore, the length of the platform is 900 - 400 = 500 meters.
Solved, answered and explained.
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To see other similar solved problem, look into the lessons
- A train passing a telegraph post and passing a bridge
- A train passing a platform
- A train passing through a tunnel
- A light-rail train passing a walking person
- A train passing another train
in this site.
Come again to the forum soon to learn something new (!)