Answer: Let's call the speed of the airplane in still air "x".
Since the airplane takes 7 hours to fly 2800 kilometers against the wind, its speed against the wind is 2800/7 = 400 km/hr.
Since the return trip with the wind takes only 4 hours, the speed of the wind must be 400 - x.
The total speed of the airplane on the return trip is x + (400 - x) = 400 km/hr.
So, we have:
400 = x + (400 - x)
Expanding the right side:
400 = 400
Therefore, x = 400 km/hr, which is the speed of the airplane in still air.
Explanation: