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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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re(2): 5,5,9--7,7,9--7,7,13 | Comment 51 of 89 |
(In reply to re: 5,5,9--7,7,9--7,7,13 by pcbouhid)

Sorry. I always initially think of nine as prime and have to kick myself and think no it's not.  It is probably because I am listing 1,3,5,7 and of course I next think 9.  But in another post I just read, Charlie? says that 1 is not considered prime.  I am sure he is right.  So now I have some more rethinking to do about prime numbers.  And of course 7,7,13 adds up to 27 = 3^3, OOH higher level math.  I figured I was worong but I always hate when I am.  I have been out of school way too long.


  Posted by john on 2005-03-14 17:42:51
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