Date: Mar 11, 2013 3:59 PM
Author: Robert Hansen
Subject: Re: Please help me with the following question


On Mar 11, 2013, at 11:49 AM, Joe Niederberger <niederberger@comcast.net> wrote:

>> At the same time I knew that the final step had to involve 3 coins or less
>
> Except that's not true.


The balanced strategy gives

1. AAAA BBBB CCCC <- 3 groups of 4 coins.
2. ABBB CCCB <- This is the next weighing if in step 1 AAAA != BBBB (otherwise it is trivial)
3. AB or BBB <- this can be resolved in one weighing AND also knowing the result of step 2.

You cannot end up with a group of 4 coins in step 3 and in one more weighing know which single coin is counterfeit, even if you know which way it leans.

Bob Hansen