Mark wrote:
Then the answer is not so obvious...

Ice/water at 32 or ice at -20?

Depends. How many square inches of contact surface? I bet 100 square inches
of 32F water will cool to 35F faster than 3 square inches of -20F ice.
Especially so if the ice forms a skin of water between ice and beer can,
limiting the interface temperature to 32F.

--
If you really believe carbon dioxide causes global warming,
you should stop exhaling.