Talk:Puzzles/Logic puzzles/Getting out of Prison

The question you ask is "Which door would the other guard say leads to freedom?"

Whatever he says, take the other door.

Nanobug 21:10, 10 Oct 2003 (UTC)
 * If the guard you asks lies, he will lie about what the other guard would say (which would be the truth).
 * So he will tell you the wrong door.
 * If the guard you asks tells the truth, he will repeat exactly what the other guard will say (which would be a lie).
 * So he will also tell you the wrong door.


 * Excellent, Nanobug. Your question does makes sure that we definitely will get a lie, because it involves both guards. Thomas

Alternate question
Assuming that the guards are restricted to answering 'left' or 'right', the following would alos be a solution:

"If I asked you which door leads to freedom, how would you answer?"

The Truth telling guard would answer truthfully and you know the correct door. The Lie telling Guard would normally have told a lie, but he must lie about his lie, therefore also revealing the correct door.

This works due to the simple principle that a double positive yeilds a positive, while a double negative yeilds a positive.