All about flooble | fun stuff | Get a free chatterbox | Free JavaScript | Avatars    
perplexus dot info

Home > Logic
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.)
5,5,9--7,7,9--7,7,13 | Comment 46 of 89 |

I see a lot of discussion around what the person (B) who saw 5,5 thinks without examining the possibility that he has a 9.  It seems everyone is still only thinking the 3,5,7,11.  If B only sees 5,5, he can not eliminate it down to the 3,5,7,11 possibility that I can by seeing 5,7.

Similarly person (C) who sees 7,7...could have a 9 or a 13.

As I am the only one considering this I may be wrong.  But can anyone point out why, If i am indeed wrong?

  Posted by john on 2005-03-14 15:24:47
Please log in:
Remember me:
Sign up! | Forgot password

Search body:
Forums (0)
Newest Problems
Random Problem
FAQ | About This Site
Site Statistics
New Comments (2)
Unsolved Problems
Top Rated Problems
This month's top
Most Commented On

Copyright © 2002 - 2021 by Animus Pactum Consulting. All rights reserved. Privacy Information