I will use 'hr' to mean hour and 'hrs' to mean hours.
The airplane flies:
200 miles per hr
400 miles per hr
600 miles per hr
Average speed = Total distance ÷ total time
= (200 + 400 + 600)miles ÷ 3hrs = 400miles/hr
Time = Distance ÷ Average speed
= 2000miles ÷ 400miles/hr = 5hrs
