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Old 22nd October 2013, 18:11   #136
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Default Re: The Riddle & Puzzle Thread

As part of its 25th anniversary, physicsworld.com has released the fourth puzzle today. The fifth and the final puzzle will be out next Tuesday. Here is the link for all the puzzle enthusiasts.

http://blog.physicsworld.com/categor...-at-25-puzzle/
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Old 22nd October 2013, 20:22   #137
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This is a puzzle I read in a novel from the 80s.

This puzzle is given to 3 people, first to solve it is the winner. Imagine yourself as one of the people.

There are 5 stickers, 2 are black, 3 are red. All three are asked to close their eyes and each gets a sticker put on their forehead. Now you are allowed to open your eyes and see each other. After you looked at each other, the stickers are removed in the same fashion.

Your observation: You saw other two having red sticker on their forehead.
Your mission: Deduce the color of the sticker that was on your head.
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Old 22nd October 2013, 23:40   #138
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Originally Posted by Samurai View Post
This is a puzzle I read in a novel from the 80s.

This puzzle is given to 3 people, first to solve it is the winner. Imagine yourself as one of the people.

There are 5 stickers, 2 are black, 3 are red. All three are asked to close their eyes and each gets a sticker put on their forehead. Now you are allowed to open your eyes and see each other. After you looked at each other, the stickers are removed in the same fashion.

Your observation: You saw other two having red sticker on their forehead.
Your mission: Deduce the color of the sticker that was on your head.
Red. If I take turns in being different person and achieving the same result, then I know I have to have red on my forehead too.
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Old 22nd October 2013, 23:46   #139
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You'll have to explain your answer by logical deduction.
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Old 22nd October 2013, 23:52   #140
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You'll have to explain your answer by logical deduction.

If anybody had a black, that person would've been the winner. Since we are still trying to figure out who won, all three of them have Red - the only state of equilibrium possible.

Edit: Sorry, that person wouldn't have been the winner, but the game would've still been over. Say I'm person #1. If either #2 or #3 had seen two black stickers, they would've known they have a red. So, there's at least one red sticker between #1&#2 OR #1&#3. But neither #2 nor #3 have claimed to solve the puzzle, which has to mean that I'm also wearing red (assuming that #2 and #3 have also used the same logic, of course). Dunno whether that makes sense

Last edited by Wimwian : 23rd October 2013 at 00:11.
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Old 22nd October 2013, 23:53   #141
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You'll have to explain your answer by logical deduction.
Ha ha, I am bad at explaining, let me give it a shot. Let's assume that three guys called A, B & C read the question and decide to try the scenario out. The first statement that states the observation has to be true for each one reading the question. Which means, A should see red stickers on B's and C's forehead, B should see red stickers on A's and C's forehead and C should see red stickers on A's and B's forehead. So, by assuming different roles (A,B & C) in my head, I could deduce that I have red sticker on my forehead too.

Was I correct?
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Old 23rd October 2013, 00:07   #142
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Let's say you are A and you are having black on your forehead. You see B and C are having red. Why is this not possible? Tackle that first. :-)
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Old 23rd October 2013, 00:27   #143
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Let's say you are A and you are having black on your forehead. You see B and C are having red. Why is this not possible? Tackle that first. :-)
Because if I am B and the same observation holds good, A can't have black. A and C will have red. Or I am reading the puzzle wrong?
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Old 23rd October 2013, 00:36   #144
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If A has black, both B and C are seeing a black and a red. Why is this not possible? Keep in mind you only know the observation of A. There are 5 stickers, and you don't know which ones were not used.
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Old 23rd October 2013, 01:35   #145
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If I had a black, B and C red:
B sees 1 red and 1 black
C sees 1 red and 1 black


If both of them don't call out that they solved the puzzle = they don't see 2 blacks, then both of them eventually should know that the other is seeing the same. Hence, they both should know that they have red and would have solved the puzzle.

If neither of them called out, then they are in the same state as Me and hence I got Red.
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Old 23rd October 2013, 09:41   #146
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Bingo! You got it. You have eliminated the possibility that you have black. So you can only have red.
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Old 23rd October 2013, 14:50   #147
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Bingo! You got it. You have eliminated the possibility that you have black. So you can only have red.
Good puzzle - kept me awake late in the night thanks for sharing!
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