The Prisoners & The Light Switch Riddle
July 26, 2012 Leave a Comment
The warden at the Prison for Creative and Unusual Punishment places a high value on intelligence, believing that intelligent people should be integrated into society no matter what their past misdeeds. So rather than holding parole hearings, the warden routinely subjects his prisoners to tests of intelligence. If they pass they are set free. If they fail, they are usually executed.
For the next test, each of 20 prisoners will be placed in solitary confinement. Every once in a while, a prisoner’s name will get drawn from a hat and he will get to visit the Rec Room. The Rec Room is no different from any of the barren concrete confinement cells except it has a light switch on the wall that isn’t wired to anything. The prisoner is allowed to flip that switch to his heart’s content for a minute, then has to go back to solitary. Then after another random length of time – maybe 5 minutes, maybe 5 days or more, another random prisoner will get selected to go to the Rec Room. Note that the same prisoner might be selected multiple times in a row. This will continue until a prisoner can tell a guard that everyone has been to the Rec Room at least once. At that point, all of the prisoners will be set free. If he’s wrong, though, everyone will get executed.
So before the test happens, the 20 prisoners get to have one last party/meeting to coordinate how they are going to have someone know when everyone has been to the Rec Room. After this meeting, however, there will be no communication between them.
What should they do?