Aplane flies 720 miles with the wind in 3 hours. the return trip against the wind takes 4 hours. what is the speed of the wind and the speed of the plane in still air?

Let p and w represent the speed of the plane and the speed of the wind, respectively.
.. speed = distance/time
.. p +w = 720/3 = 240
.. p -w = 720/4 = 180

Add the two equations to eliminate w.
.. 2p = 420
.. p = 210
.. w = p -180 = 30

The speed of the wind is 30 mph.
The speed of the plane in still air is 210 mph.