Case 1: The planes fly towards each other. Distance covered by plane 1 in 1 hour = 550 km. Distance covered by plane 2 in 1 hour = 750 km. Total distance covered towards each other = 550 km + 750 km = 1300 km. Remaining distance = 3900 km - 1300 km = 2600 km.
Case 2: The planes fly away from each other. Distance between them after 1 hour = 3900 km + 550 km + 750 km = 5200 km.
Case 3: One plane overtakes the other (assuming they start from opposite ends and fly in the same direction). The relative speed is 750 km/h - 550 km/h = 200 km/h. The distance between them will be 3900 km - 200 km = 3700 km.
Possible distances are 2600 km, 5200 km, and 3700 km.