Question 331595
When you have a headwind, you have to subtract the wind's speed from the plane's velocity, because it's slowing it down:


Let Vh = Velocity of plane with headwind, and Vp=Velocity of plane with no wind.


Vh = Vp - 40


Let Vt = Velocity of plane with tailwind


Vt = Vp + 40


We know that d=vt (d=distance, v=velocity (speed), and t=time) and also that the distance each way is exactly the same. Therefore we can set the Distances equal to each other:


D = D
Vt*time = Vh * time


(Vp+40)2 = (Vp-40)3
2Vp+80 = 3Vp -120
200 = Vp


So the speed of the plane in still air is 200mph.


Hope that helps. Let me know if you're still confused.