Why seven shuffles are enough to mix a deck of cards
30seconds
A 52-card deck can be put in about 8 × 10^67 different orders, yet a few quick shuffles leave a lot of the old order behind.
In 1992 Dave Bayer and Persi Diaconis worked out exactly how random a deck is after each riffle shuffle. After four shuffles the deck is still easy to tell apart from a random one; then randomness arrives almost all at once. Seven riffles is the point where a deck finally looks close to random. Shuffle a virtual deck yourself and watch the pattern dissolve.
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