Predicting the future accurately is most useful in betting games--the stock market comes to mind. Unfortunately, perfect oracles are hard to come by (the stock market comes to mind, again). This puzzle considers how to take advantage of the flaky oracles one is likely to find.

You have $100 to start with and 10 bets to make. Each bet turns on the result of a coin flip. The oracle will tell you which way the coin will fall but may lie on just one occasion and may do so after seeing your bet for that flip. You can find a counterparty who will give you even odds on any bet you make, so placing an x-dollar bet means he will return 2x dollars to you if the oracle tells the truth about that flip and will pocket your bet if the oracle lies. How do you end up with the greatest possible final amount, no matter when the oracle chooses to lie?

This article was originally published with the title The Delphi Flip.

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