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Prime Perimeter (Posted on 2005-03-11) Difficulty: 3 of 5

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.

What is the number written on your forehead?

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

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Solution prime perimeter | Comment 68 of 89 |
I think that i  have found  the answer to  the riddle:prime  perimeter!

it say that three numbers are listed  on each others heads.Two of the numbers  are  5 and 7.It also  says that  the  total perimeter  is a prime  number.  I think that  shape  is a right  triangle ,  and not   an equalateral. The numbers imply that, since they are not equal.5 plus seven equals 12. the bottom halve will end up as 5 if you do the math . 5+5+7=17, which just happens to be a prime number! So....17  is the final answer!!
  Posted by jason on 2005-03-26 02:58:33
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