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. let x be the maximum speed of the plane and y be the speed pf the wind. what is the speed of the plane with no wind

x = velocity of plane
y = velocity of wind

x - y = 1680/5 = 336 mph .....(1) [ plane flying against the wind]
&
x + y = 1680/4 = 420 mph....(2) [ plane flying with the wind]

Adding (1) and (2) to eliminate y gives

2x = 756
x = 378 mph is the speed of the plane with no wind.