Question 58613
Let the speed of the plane in calm air be x mph
With the wind speed of the plane = (x+20)mph
Against the wind speed of the plane = (x-20) mph
Time taken to travel 480 miles with the wind = 480 / (x + 20) hrs
 Time taken to travel 480 miles against the wind = 480 / (x - 20) hrs
Therefore according to the problem,

480/(x-20)  - 480/ (x+20) = 1

==> 480x + 9600 -480x + 9600 = x^2 - 400

==> 19600 = x^2

==> x = 140

Therefore the speed of the plane = 140 mph