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Topic: A nice math puzzle
Message: Posted by: balducci (Aug 24, 2006 02:06AM)
I came across this puzzle in a recent issue of The Actuarial Review (a newsletter for casualty actuaries). This newsletter has published quite a number of good math puzzles, and some math related magic tricks, over the years. This puzzle isn't a magic trick as such, but I think the method might be useful in an effect, somewhere, somehow.

Here's a link to the puzzle:


I don't want to spoil your fun right away, so I'll post the solution later ... although web savvy people should be able to find it on their own, without too much difficulty.
Message: Posted by: Nir Dahan (Aug 25, 2006 07:38PM)
Small question.
Are any of the people allowed to change the position of the cards behind the curtains they looked at?

Message: Posted by: balducci (Aug 25, 2006 10:55PM)
No. That would be a different puzzle, which might be worth exploring in its own right. But it's not allowed in this one.
Message: Posted by: stanalger (Aug 27, 2006 05:39PM)
Great puzzle, balducci!
At first thought, a solution didn't seem possible....
Message: Posted by: Carlos the Great (Aug 28, 2006 01:04PM)
Right, okay, so I tried and tried and gave up and found the solution (you are correct, the web savvy should be able to find the answer in 10 seconds since it took me 20).

That was painful, I had to work through it before I saw what was happening and I'm still a bit... irritated, lol.

Message: Posted by: Nir Dahan (Aug 28, 2006 04:13PM)
Try also looking here:

Message: Posted by: Nir Dahan (Aug 30, 2006 05:51AM)
I managed to reach a probability of 5/12 which is 41.6666666%
I assume it to be the maximum. or anybody has a better solution?
Message: Posted by: arnsfelt (Sep 3, 2006 08:26AM)
The puzzle actually originates from a question in theoretical computer science.
It was first stated by Peter Bro Miltersen and published in the paper "The Cell Probe Complexity of Succinct Data Structures" by Anna Gál and Peter Bro Miltersen. ([url]www.daimi.au.dk/~bromille/Papers/succinct.pdf[/url]). The solution to the puzzle was found by Sven Skyum.