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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)

Comments: ( Back to comment list | You must be logged in to post comments.)
re: centering the point | Comment 38 of 89 |
(In reply to centering the point by armando)


1) it's tacitly said that the three numbers are primes.

2) it's tacitly said that the numbers could be sides of a triangle.

3) it's tacitly said that the perimeter is a prime.

4) it's tacitly said that the two others could not guess theirs numbers.

So, if my number could only be 3 or 5 or 7 or 11, and I eliminate the first three, it could only be 11. PERIOD!!  

  Posted by pcbouhid on 2005-03-13 20:54:36
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