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Lateral Thinking #18- The 20 Prisoners -

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Puzzle Image The warden of a small minimum security prison believes that everyone deserves a second chance. He rounds up the 20 prisoners in his prison and offers to release them if they can solve his puzzle.

Each of the 20 prisoners will be placed alone in solitary confinement. The warden will then draw a name at random and bring the prisoner to an empty building. Inside the building is a light switch that does nothing. The prisoner can flip the switch up or down if they like, after which they'll be escorted back to their cell. Sometime later, the warden will bring another prisoner at random to the empty building with the light switch. Once again, the prisoner can flip the switch up or down if they like, after which they'll be escorted back to their cell. Note that the same prisoner might be selected multiple times before some prisoners are selected at all.

This will continue until one of the prisoners tells the warden that all 20 of them have been to the empty building with the light switch at least once. If the prisoner is right, they will all be released. If the prisoner is wrong, the group loses their chance for immediate release. "Oh, one more thing, the light switch is currently in the DOWN position" the warden tells the prisoners.

The prisoners have one hour to come up with a plan to solve the puzzle. When the hour is up, they will be escorted to their solitary confinement and there will be no further communication of any kind between them. Can you figure out the answer? Note that this is NOT a trick question, the answer lies in some kind of coordinated switch-flipping strategy.
Rating: 2.6 on 5 (14 votes)
 
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