Question 890887
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Let's forget about the technical definitions of the terms air speed and ground speed.  This is a problem for learning how to do math; it is clear what the intended meaning is.<br>
Flying with the wind, the plane flew for 2 hours at 400mph; that is 2*400 = 800 miles.<br>
Flying the same distance of 800 miles back, the trip took 2.5 hours.  The speed is 800/2.5 = 320mph.<br>
The difference in the two speeds is the speed of the wind: 400-320 = 80mph.<br>