How to Make a Fair Coin Flip With an Unfair Coin

The coin flip, the ultimate 50-50 choice, is actually a little biased. According to a Stanford study, even a fair coin isabout 51% likely to land on the same face it started on. And if you spin instead of flipping, even a slightly weighted coin is much more likely to land on its heavier side. (A spun penny lands tails-up 80% of the time.) But there’s a cool math trick for getting a fair result from an unfair coin.

The Fermat’s Library Twitter account, a grab-bag of mathematical and scientific curiosities, posted a simple solution: Instead of picking heads or tails, pick between heads-tails and tails-heads. As long as you make the same flip each time, HT and TH have equal probabilities.

Using this technique, you could flip practically any object that can land on two different sides, no matter how often it lands on one side over the other. Or really any random binary process that you can’t influence.

Follow Fermat’s Library for more clever math tricks, like how to fold any circle into an ellipse, or a cheat sheet for all the trigonometric functions.

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https://lifehacker.com/how-to-make-a-fair-coin-flip-with-an-unfair-coin-1822413841