I have a bag containing the digits 0 through 9, and used six of them to stick two different threedigit perfect squares on the foreheads of Paul and George. Both Paul and George know this fact, but each one can see only the other's number.
I ask Paul, "How many of the digits remaining in my bag can you exactly tell me?"
Paul replies, "Three."
If I now ask the same question to George, what should he reply?
(In reply to
George's answer by Bob Smith)
So that actually means he only knows 3: the fourth might be either a 1 or a 7. He doesn't know all 4.

Posted by Charlie
on 20050804 18:05:10 