i can do the distance=rate x time problems well enough but setting this one up has utterly baffled me.

Hazel and Emily fly from atlanta to San diego. the flight to sandiego is against the wind and takes 4 hours. the return flight with the wind takes 3.5 hours. if the speed of the wind is 40 mph find the speed of the plane in still air.

i know the table you do where you set it up but im just utterly baffled as how to set this one up to find the speed of the plane in still air. something to do with resistence...uuhhh...help?