Note: Read this problem carefully, because it's completely different from the original.

As before, 100 prisoners are put into solitary cells, and there's a room with a light bulb. (No prisoner can see the light bulb from his or her own cell.) Every night, the warden picks a prisoner at random, and that prisoner goes to the living room. While there, the prisoner can toggle the bulb if he or she wishes. but this time, the prisoner needs to assert that he knows, which prisoner was in the living room before him. If the assertion is false, all 100 prisoners will be shot. However, if it is indeed true, all prisoners are set free. 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, to discuss a plan.

But, the prisoners know that after that night, when they will go back to their solitary cells. the warden will choose one prisoner secretly (and this time, not randomly) and will kill him.

What plan should they agree on, so that eventually, someone will make a correct assertion?

Though it might be tricky for prisoners to keep track of the day of the week exactly for hundreds of days. An easier way to do this would be to have 2 prisoners alternate being A every X number of days. X would be something like 20 to be easy enough to track, but  small enough not to have to wait 3 years to get out. Of course this solution is not guarenteed to be 100% because the warden could just be a real jerk and never pick A.

Posted by George on 2007-03-07 12:27:13