At maximum speed, an airplane travels 1680 miles against the wind in 5 hours. flying with the wind, the plane can travel the same distance in 4 hours.

D=vt

1680=5(p-w)

p-w=336

1680=4(p+w)

p+w=420

p-w+p+w=336+420

2p=756

p=378 mph, and since w=420-p

w=42 mph

So the plane's velocity in still air is 378 mph and the wind speed is 42mph.