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Bob's Hat (Posted on 2002-06-13) |
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Three friends, Alan, Bob and Cecil, sit down at a park bench. Each of them is a Knight or Liar and each knows what the other two are.
Forgetful Bob, well known for his flamboyant hats that are either red, white or blue, asks Alan: "What colour hat am I wearing today?"
Alan replies: "There is a chance Cecil would tell you it's blue. If you saw it, there is a chance you would say it is white."
Bob sighs, "I should have asked Cecil - he always tells me the truth..."
What colour hat is Bob wearing?
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Submitted by Nick Reed
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Rating: 3.6667 (15 votes)
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Solution:
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(Hide)
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Let's take the statements one at a time:
1) A says "There is a chance C would tell you it's blue"
A would only say this if:
- A is a Knight, C is a Knight, Hat is Blue
- A is a Knight, C is a Liar, Hat is not Blue
- A is a Liar, C is a Knight, Hat is not Blue
- A is a Liar, C is a Liar, Hat is Blue
2) A says to B "If you saw it, there is a chance you would say it is white"
A would only say this if:
- A is a Knight, B is a Knight, Hat is White
- A is a Knight, B is a Liar, Hat is not White
- A is a Liar, B is a Knight, Hat is not White
- A is a Liar, B is a Liar, Hat is White
Combining these with the options from point 1 results in:
- A is a Knight, B is a Knight, C is a Liar, Hat is White
- A is a Knight, B is a Liar, C is a Knight, Hat is Blue
- A is a Knight, B is a Liar, C is a Liar, Hat is Red (not Blue or White)
- A is a Liar, B is a Knight, C is a Knight, Hat is Red (not Blue or White)
- A is a Liar, B is a Knight, C is a Liar, Hat is Blue
- A is a Liar, B is a Liar, C is a Knight, Hat is White
3) B says "C always tells me the truth"
B would only say this if:
- B and C are both Knights
- B and C are both Liars
Combining these with the options from point 2 results in:
- A is a Knight, B is a Liar, C is a Liar, Hat is Red
- A is a Liar, B is a Knight, C is a Knight, Hat is Red
So, although we don't know who is who, we know the hat is Red. |
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