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This question already has an answer here:

There are two boxes in front of you. One box has an octopus inside it and the other box does not. Box A has owner A standing in front of it and Box B has owner B standing in front of it. One of the owners ALWAYS tells the truth and the other owner ALWAYS lies. Both owner A and owner B know which Box has the octopus inside it. Both owner A and owner B know whether or not the other is a truth-teller or a liar. What should you ask owner A in order to find out which box has the octopus behind it?

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marked as duplicate by Oray, Sconibulus, Sid, Gamow, JonMark Perry Oct 2 '17 at 14:20

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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You should ask A:

what would B answer me if I ask him if the octopus is in your box?

the answer will be

a lie! So you will know which box to open

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