I’m following your logic as well. That’s an entirely different perception on the speed.
Sadly, speed does matter, as that it is part of the question. Equal speed of the treadmill to the plane. But, what part of the plane? You just summed it up. Two different parts working with different factors on perceptual speed.
But, taking the plane as an object as a whole and not splitting it into wheels and body, the perception is different.
EDIT:
Wait, I read yours again. Your math doesnt add up. If a plane is moving 150 MPH forward in relation to actual forward movement, and the treadmill is going 500 MPH backwards… the wheels are going 750 MPH, and not equal. You add the two together. They may not have the same forces, and don’t have to be equal but they do constitute the final speed of the wheels on the ground. All three of your numbers are not equal, so you broke the question by not having equal speeds.