16 Jun '04 17:01>2 edits
There are 3 prisoners in a prison. Let’s call them A, B, and C. Tomorrow, 2 of them will be executed, but the prisoners don’t know which of them have been chosen. Prisoner A reasons that his chances of survival are 1/3 (a third). He then gets bored in his cell and goes to chat with a guard. “Tell me”, he says, “will I be executed tomorrow?” “I’m sorry,” the guard replies, “I am not allowed to tell you that.” “OK”, says Prisoner A, “you needn’t tell me anything about myself. But one of the other two, either B or C is sure to be executed. Perhaps both of them but, in any case, at least one of them.” “That’s right”, confirms the guard. “Very well”, says Prisoner A, “tell me the name of one person (B or C) who is to be executed.” “All right”, answers the guard, “B will be executed tomorrow.” On hearing this news Prisoner A feels much more cheerful. He reasons that tomorrow either he or Prisoner C will be executed. And that therefore his chances of survival have increased from 1/3 to 1/2. Is he right?