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 Prime Perimeter (Posted on 2005-03-11)

You and two other people have numbers written on your foreheads. You are all told that the three numbers are primes and that they form the sides of a triangle with a prime perimeter. You see 5 and 7 on the other two heads and both of the other people agree that they cannot deduce the number on their own foreheads.

 See The Solution Submitted by Erik O. Rating: 2.8235 (17 votes)

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 re(3): THERE IS NO SOLUTION! | Comment 82 of 89 |
(In reply to re(2): THERE IS NO SOLUTION! by jon)

"if the 5 thinks hes a five, hes looking at 7 and 17. His equation is 5+7+17=29, which is prime. but he can assume that he's a 7 along with the 7 and have the equation 7+7+17=31."

You can't draw a triangle with sides 5, 7, and 17.

"the same thing goes for #7. When the seven thinks he's a seven the only thing he sees is  the 5 and the 17, so his equations can be 7+5+17=29 or 19+17+5=41."

You can't draw a triangle with sides 5, 7, and 17.

And I wasn't saying they would tell the other what they saw, I was just saying they could gather information from the other's silence.

 Posted by Dustin on 2005-04-09 02:55:50

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