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Un-biasing a biased coin

Puzzle: You and a friend are sharing a car. Neither of you wants to drive. Your friend suggests you flip a coin for it, and offers up a coin. You suspect it might be biased. How might you still use the coin to ensure a fair decision?

Answer: Flip the coin twice. Instead of deciding between heads and tails, decide between HT and TH. These outcomes are equally likely, since the two flips are independent. If you get HH or TT, just flip again.

Originally published on Medium.

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