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Two guards

433 bytes added, 09:12, 17 October 2010
Alternative answer (Source: The Chicken from Minsk by Yuri B Chernyak & Robert M Rose)
{{Hint| It is impossible (and unnecessary) to determine which guard is a truth-teller and which is a liar.}}
{{Answer| Ask either guard, "If I were to ask the other guard which door leads to safety, what would he say?" Go through the ''other'' door.}}
Alternative answer: Ask either guard "Is the truthful guard standing in front of the door that leads to death?" If the answer is no, go through that door; if yes, go through the other one.}} {{Solution| Why does this work? Let's consider the two cases: (1) the guard you ask always lies or (2) the guard you ask always tells the truth. In case 1, the other guard always tells the truth, so if you asked him which door leads to safety, he would tell you the correct door. The guard you asked must lie about that though, so he will tell you the other door. In case 2, the other guard always lies, so would tell you the wrong door, and the first guard faithfully conveys this information to you. In either case, you will be told the wrong door, so going through the other door is the correct play. The alternative answer is less well known, perhaps because it doesn't work for versions in which the guards don't stand directly in front of the doors. (Also because it's never appeared on television.) But it works on a very similar principle.}}
[[Category: Liar puzzles]]
[[Category: Logic]]

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