I know this word problem involves systems of equations, but I have never seen one similar to this before, and I don't have the slightest idea where to start. I can solve it when it's set up, I just don't know how to set it up.

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Two planes leave Pittsburgh and Philadelphia at the same time, each going to the other city. One plane flies 25 mph faster than the other. Find the air speed of each plane if the cities are 275 miles apart and the planes pass each other after 40 minutes of flying time.

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Thank you for any and all help.