Problem:

Three prisoners are given the chance to go free if they can solve a puzzle. They stand in a line where each can only see in front of them. They are told that they are wearing hats that are either black or white, and at least one of them is wearing a black hat. They must guess their own hat color without turning around or communicating. How can they guarantee their freedom?

Scroll to the bottom of the page for the answer

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Solution:

The last person in the line sees the two hats in front. If both hats are white, they say "black," knowing their hat must be black as there is at least one black hat. If the hats are mixed or both black, they say nothing. The second person uses the first person's response to deduce their hat colour.