If they run in opposite directions from the start, they are actually running towards each other on a circular track.
This means that the question is asking how long it takes to cover 200 meters if one person is running at 5 m/s and the other is running in the opposite direction at 3 m/s.
Since they are approaching each other at 8 m/s in total, the time taken to cover 200 meters is 200/8 = 25 seconds.