Newsgroups: rec.puzzles
From: hoey@aic.nrl.navy.mil (Dan Hoey)
Date: 30 Sep 1994 23:03:14 GMT
Subject: Re: An interesting riddle

hoey@aic.nrl.navy.mil (I) wrote:

> ObPuzzle:  What is the minimum number of known good coins needed
> to find the one odd coin from (3^W-1)/2 suspect coins, and to find
> whether it is heavy or light, in W weighings on a balance scale?

Jim Saxe was good enough to point out to me that this is in fact
covered in the FAQ.  The answer is one.

I suppose I need to trot out a more challenging ObPuzzle.  Suppose we
are instead charged 1 unit for the each coin we put on the balance,
each time we weigh it.  What is a minimum-cost solution for weighing
the bad coin then?  What if are charged 1 unit per coin, plus K units
for each weighing?

Dan Hoey
Hoey@AIC.NRL.Navy.Mil