The speed of the plane will be 200 knots.
You do 800/4 because you are trying to find the speed of the airplane.
If you take the speed of the plane and divide the distance by the speed and then followed by the 5 and 4, you will get 800/200/4/5
800/200
=400
400/4
=100
100/5
=20
The speed of the wind is 20 knots.