This mathematical trick, which may well be the best mathematical magic trick ever devised, was not part of my Pi Day magic show. However, it should have been. Here’s a description of the trick, modified from the description at http://mathoverflow.net/questions/20667/generalization-of-finch-cheneys-5-card-trick:
The magician walks out of the room. A volunteer from the crowd chooses any five cards at random from a deck, and hands them to your assistant so that nobody else can see them. The assistant glances at them briefly and hands one card back, which the volunteer then places face down on the table to one side. The assistant quickly place the remaining four cards face up on the table, in a row from left to right. After all of this is completed, the magician re-enters the room, inspects the faces of the four cards, and promptly names the hidden fifth card.
In turns out that the trick is a clever application of permutations (there are 
Unlike the other tricks in this series, the Fitch-Cheney 5-Card Trick requires a well-trained assistant (or a smartphone app that plays the role of the assistant).
A great description of how this trick works can be found at Math With Bad Drawings. For a deeper look at some of the mathematics behind this trick, I give the following references:
- https://possiblywrong.wordpress.com/2011/05/29/generalizing-fitch-cheneys-five-card-trick/
- http://www.apprendre-en-ligne.net/crypto/magie/card.pdf
- http://blue.butler.edu/~phenders/sigcse2005/niftyworkshop/Handouts/cardTrick.pdf
- http://www.jstor.org/stable/25678404?seq=1#page_scan_tab_contents (requires JSTOR access)
Mean Green Math 
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