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100 Prisoners and 100 Boxes

Puzzle

A room has 100 boxes labeled 1 thru 100. The names of 100 prisoners have been placed in these boxes by the warden. The prisoners shall visit the room one by one. Each prisoner is allowed to inspect the contents of at most 50 boxes, one after the other and leave the room with no communication with other prisoners. If the prisoner discovers his own name in the boxes he inspects, he is released. The prisoners are allowed to collude before hand and devise a strategy to maximize the chances of releasing each and every prisoner. What is their strategy?

Source

A colleague at Google asked me this puzzle in 2008.

Solution

A big rectangle is composed of smaller non-overlapping rectangles, each having integer width or integer height or both. Prove that the big rectangle enjoys the same property.

There are 100 prisoners in solitary cells. There's a central living room with one light bulb; this bulb is initially off. No prisoner can see the light bulb from his or her own cell. Everyday, the warden picks a prisoner equally at random, and that prisoner visits the living room. While there, the prisoner can toggle the bulb if he or she wishes. Also, the prisoner has the option of asserting that all 100 prisoners have been to the living room by now. If this assertion is false, all 100 prisoners are shot. However, if it is indeed true, all prisoners are set free and inducted into MENSA, since the world could always use more smart people. Thus, the assertion should only be made if the prisoner is 100% certain of its validity. The prisoners are allowed to get together one night in the courtyard, to discuss a plan. What plan should they agree on, so that eventually, someone will make a correct assertion?

18 Aug 2008
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