You have a square glass panel with 24 cm sides and a ring with 5 cm internal diameter.
Cut the glass in 4 identical pieces such that you can slide them through the ring.

(In reply to re: Thoughts by brianjn)

The external diameter matters inasmuch as the thickness of the ring matters.  If you think of the ring as cylinder that's been bent into a circle and attached top-to-bottom, the diameter of the cylinder you started with does make a difference when we try to move the ring around a corner.  This is because a thick ring won't be able to fit snug against the inside of the corner. 

Also, depending on the shape of the pieces you get, space may be limited by it's geometry.  For example, in JEFF's solution, the ring can't be more than 3" thick, or it won't fit between the sides of the 'horseshoes'.    

Posted by josh on 2005-07-06 00:18:20