A plane can travel 640 miles against the wind in the same time it can travel 800 miles with the wind. If the plane can fly 180 mph in calm air, what is the speed of the wind?

(plane velocity + wind velocity) * time = 800

(plane velocity - wind velocity) * time = 640

We don't know the time but we do know it is the same so we can say:

(plane velocity + wind velocity) / (plane velocity - wind velocity) = (800/640)

(180 + wv) / (180 - wv) = 1.25

(180 + wv) = 225 - 1.25wv

45 = 2.25 wv

wind velocity = 20 mph