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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(3): full solution as understood | Comment 25 of 89 |
(In reply to re(2): full solution as understood by Charlie)

I agree with Charlie  on scenario 1.

but scenario 2. is not possible because if C were seeing 3 and 7 then 3 is not a possibility for C. As sum of two sides of a triangle is greater than third side.

so scenario 1. is still possible.

Lets check scnario 3. may be it is not possible and our number is 5 not 11.

What do you think Charlie and pcbouhid ?

Edited on March 12, 2005, 4:12 am

Edited on March 12, 2005, 4:12 am
  Posted by Sachin on 2005-03-12 04:11:29

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