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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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 No Subject | Comment 54 of 89 |
assume you have 11 on your head, so the guy with the 7 on his head sees 11 and 5, he adds 11 and 5 and gets 16. The next prime greater than 16 is 17, however the difference 1 is not prime, the next prime thereafter is 19,  the difference 19-16=3, could be on your head (blowing the exclusivity of the initial assumption) . The prime greater than 19 is 23, which is 7 greater than 16, therefore 11+5+7=23.

the solution could be 3 as well as 11.

 Posted by emdad on 2005-03-14 22:04:02

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