The solution for this problem is:
Let x = speed of wind
Speed of plane with the wind = x + 100
Speed of plane against the wind = 100 -xÂ
 We will be using the formula for distance which is (Rate)(Time), getting the formula for time would be distance/rate Time to travel 600 miles with the wind = Time to travel 400 miles against the wind 600/(x + 100) = 400/(100 - x)
 400(x + 100) = 600(100 - x)
 400x + 40000 = 60000 - 600x
 1000x = 20000
x = 20000/1000
 x = 20 mph