Good insight, and good use of the extreme example. I guess the question is sloppy as newtboy said. I got hung up on "they have the same mass" which would definitely mean they would be balanced with no air.

I'm not sure where air comes into play for problem one. There's no "air" factor in the gravity equation. F=GM₁M₂/(r^2)

But yeah, by definition, you need to use the equation to get the 20pi answer...

Tapered wheels.