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Seven traders, red or blue hats, win seven times in eight

Asked at Jane Street, Citadel, Two Sigma

Seven traders each get a red or blue hat decided by an independent fair coin. Each sees the other six hats but not their own. At a signal, every trader simultaneously either says "red," says "blue," or passes. The team wins if at least one person guesses and every guess made is correct; a single wrong guess loses.

If just one person guesses and the rest pass, the team wins with probability 1/21/2. That feels like a ceiling.

Devise a strategy that wins with probability 7/87/8.

Your answer

This one is open-ended. Work it through, then check your reasoning against the full solution.

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