Thursday, April 22, 2010

Prisoner Puzzle

A prisoner who has commited a crime is before a judge. The judge sentences the prisoner to death by hanging, but adds a cruel twist to the sentence. The prisoner is to be hanged on one of the following seven days – but it must be a surprise which day it is. The prisoner is not allowed to know. Returning to his cell the prisoner is a bit disturbed. His lawyer tells him not to worry.

“Look,” the lawyer says, “they can’t hang you at all now. The judge has made it a condition that you must be surprised. But think about it. If you make it to Saturday without being hung then Sunday is the last day they could do it. But then it wouldn’t be a surprise, would it? So that makes Saturday the last day they could possibly hang you. But hang on – if Saturday is the last possible day then it also can’t be a surprise to hang you then. So that makes Friday the last possible day – and so on back through all the days of the week. Therefore, they can’t possibly hang you without breaking the judge’s orders.

The prisoner is comforted by this line of reasoning and stops worrying about the prospect of being hung at all. When on Wednesday he is taken from his cell and hung.