GPT 5.2 Chat:
> There isn’t enough information to determine the inner diameter *for sure*.
All we’re told is:
- The coaster has a diameter of 9.
- The glass sits on the coaster.
- The glass wall thickness is 1.
Nothing explicitly states that the *outer diameter of the glass equals the coaster’s diameter*. The glass could be smaller than the coaster, which is very common.
*However*, if we make the usual implicit assumption in this kind of problem—that the coaster’s diameter matches the *outer diameter of the glass base*—then:
- Outer diameter of glass = 9
- Wall thickness = 1 on each side
So:
\[
\text{Inner diameter} = 9 - 2(1) = 7
\]
*Inner diameter = 7 (same units as the coaster)*
Without that assumption, the problem is underdetermined.