Answer:
The speed of plane is 378 mph in still air.
Step-by-step explanation:
Let x be the maximum speed of the plane.
Let y be the speed of the wind. 
Distance an airplane travels = 1680 miles against the wind in 5 hours 
So, equation  becomes,
 or
 or  .....(1)
      .....(1)
And with the wind, equation becomes,
 or
 or  .... (2)
    .... (2)
Solving both equations, we get;

x = 378
And x+y=420

Hence, the speed of plane is 378 mph and the speed of wind is 42 mph.