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The three prisoners and the red hats

Visual trapsLevel 3 · Intermediate · ●●●○○

Three perfectly logical prisoners are in separate cells. The guard tells them, "I'll give each of you a red or blue hat.

At least one hat is red. You can see the other two's hats through the windows, but not your own.

When someone deduces with certainty the color of your hat, they shout out their color and everyone is free. If someone shouts out the wrong color, they all die." He puts red hats on all three of them.

After a minute of silence, one of them shouts "RED!" How did you deduce it?

Hints

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  1. The cry "RED!" It is not intuition: it is elimination of the hypothesis "I am blue" using the observed silence as logical information.
  2. 2) Reference scenario with 2 red and 1 blue
  3. The one who shouts "RED" uses reasoning by levels of knowledge (induction about how many reds there are).

Solution

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Answer: The one who shouts "RED" uses reasoning by levels of knowledge (induction on how many reds there are).
1) Conceptual basis
Public rule: "at least one hat is red."
That generates a game of logical rounds:
-If someone saw two blue ones, they would instantly deduce that theirs is red.
2) Reference scenario with 2 red and 1 blue
If one prisoner were blue and the other two were red:

  • every red would see (red, blue),
  • after a first silence, each red would conclude that it cannot be blue and would deduce red.

That is to say: with exactly 2 reds, the deduction comes one round later.
3) Real case: 3 reds
Each prisoner sees two reds and reasons:

  • "If I were blue, we would be in the case of 2 reds; then the other two would have already been able to deduce it after that first silence."
  • "But that expected behavior did not occur."
  • "Then I'm not blue."

Therefore he deduces that his hat is red.
4) Conclusion
The cry "RED!" It is not intuition: it is elimination of the hypothesis "I am blue" using the observed silence as logical information.


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