Riddle courtesy of William Wu’s riddle archive.
10 prisoners are on death row. Tomorrow they will be arranged in single file, all facing one direction. The guy in the front of the line (he can’t see anything in front of him) will be called the 1st guy, and the guy in the back of the line (he can see the heads of the other nine people) will be called the 10th guy.
An executioner will then put a hat on everyone’s head; the hat will either be black or white, totally random. Prisoners cannot see the color of their own hat.
The executioner then goes to the 10th guy and asks him what color hat he is wearing; the prisoner can respond with either “black” or “white”. If what he says matches the color of the hat he’s wearing, he will live. Else, he dies. The executioner then proceeds to the 9th guy, and asks the same question, then asks the 8th guy … this continues until all of the prisoners have been queried. Assume each prisoner can hear each previous answer.
This is the night before the execution. The prisoners are allowed to get together to discuss a plan for maximizing the number of lives saved tomorrow. What is the optimal plan? How many lives can you save?
The original riddle calls for you to also generalize the plan to N prisoners and K different color hats. I haven’t done that yet.