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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: THERE IS NO SOLUTION! | Comment 80 of 89 |
(In reply to THERE IS NO SOLUTION! by jon)

Actually, Erik O. is anything but an "ameature".

You need to take your thinking a step further.

"if five thinks he's 5, then 5+ 17+ 7= 29, or if he assumes that he's a 7 then 7+17+7 = 31. Number 5 cannot know his number."

If five thinks he's 5, and seven sees a 5 and a 17, what will seven think?

"if seven thinks he's 7, then the 7+5+17=29, or if he assumes that hes a 19, then 19 +17+5=41. Number 7 Cannot know his number."

If seven thinks he's 7, and five sees a 7 and a 17, what will five think?


  Posted by Dustin on 2005-04-09 00:44:59
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