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

 Prime Perimeter (Posted on 2005-03-11)

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.

 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: Semi-Solution? | Comment 3 of 90 |
(In reply to Semi-Solution? by lauren)

"5, 7: these can't work because if either of the other two people see the same number on 2 foreheads, they'll be able to work out the answer."

I don't understand this.  If I have a 5 on my forehead also, the guy with the 7 will see two 5's, and not know if he himself has a 3 (to make 13) or a 7 (to make 17).  And the guy with the 5 on his head (besides me, in this case), would be in the same condition as myself.

Likewise, if I had a 7, the guy with the 5 wouldn't know if he had a 3 or a 5 (to make 17 or 19), and the guy with the 7 would be in my position.

 Posted by Charlie on 2005-03-11 18:26:51

 Search: Search body:
Forums (0)