Learn more about Russian war crimes in Ukraine.

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.

I just released TigYog: interactive tutorials on coding, math, crypto, science! Learn from wizards, or write your own quizzes! What’s your bag?

More by Jim

Tagged . All content copyright James Fisher 2013. This post is not associated with my employer. Found an error? Edit this page.