Answer:
87.5%
Step-by-step explanation:
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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.