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    OlympiadHardLogic7–9

    One Question, Two Guards

    Uma Pergunta, Dois Guardas

    Two doors are watched by two guards. One door leads out and one does not. One guard always tells the truth and the other always lies, but you do not know which is which. You may ask exactly one question, to exactly one guard, answerable by yes or no. What do you ask?

    Make the answer pass through both guards, so that the single lie is applied an even number of times.

    Solution

    Step 1 of 6

    1. 1.Ask either guard: “If I asked the other guard whether this door leads out, would he say yes?”