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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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 Semi-Solution? | Comment 2 of 89 |

I just started writing down prime numbers to see if they fit:

3: 3+5+7=15=not prime

5, 7: these can't work because if either of the other two people see the same number on 2 foreheads, they'll be able to work out the answer.

11: 5+7+11=23=prime, OK

the unknown side can't be any larger than 11 because then the combo of sides won't make a triangle, so this has to be the answer.

does seem too easy though.

 Posted by lauren on 2005-03-11 17:42:13

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