Let

d = 3600 miles the distance traveled in one direction

t1 = 4 hours the time of the flight with the wind

t2 = 5 hours the time of the return trip against the wind

w = the speed of the wind

v = the speed of the plane in still air

v - w = the speed of the plane flying against the wind

v + w = the speed of the plane flying with the wind

We will use the formula speed = distance/time.

Traveling with the wind:

v + w = d/t1

v + w = 3600/4

v + w = 900

Traveling against the wind:

v - w = d/t2

v - w = 3600/5

v - w = 720

We have the following system of equations:

v + w = 900

v - w = 720

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click here to see the equation solved for v and w

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v = 810 mph

w = 90 mph

The speed of the plane in still air is 810 mph.

The speed of the wind is 90 mph.