Puzzle
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Ponder This Challenge - May 2018 - Two fake coins

We have 11 coins, and exactly two of them are counterfeit with slightly smaller weights than the others. Find the two fake coins using just four weighings of a double-pan balance scale.

Supply your answer as four lines of dash-separated lists of the letters A-K corresponding to the coins.

As an example, here is a solution for the case of seven coins:
ADE-BCG
AG-BE
AC-DG

The first weighing checks coins A,D,E on the left pan vs coins B,C,G on the right side, etc.

Solution

  • Alex Fleischer sent us an ILOG solution (thanks again). It makes solving this challenge look so easy...

    Alexander Raß proved that there are two types of solution:

    ABC-DEF
BEH-CDJ
DGH-EIJ
AGI-FHJ
and
ABC-DEF
DGH-EIJ
AGI-BHJ
CDEHI-ABFGJ

Solvers

  • Bert Dobbelaere (26/04/18 09:52 PM IDT)
  • Alex Fleischer (27/04/18 01:38 PM IDT)
  • Arthur Vause (27/04/18 04:06 PM IDT)
  • Jesse Rearick (27/04/18 07:23 PM IDT)
  • Todd Will (27/04/18 07:37 PM IDT)
  • Alper Halbutogullari (28/04/18 09:25 AM IDT)
  • Eden Saig (28/04/18 04:46 PM IDT)
  • Reiner Martin (28/04/18 11:46 PM IDT)
  • JJ Rabeyrin (29/04/18 05:45 PM IDT)
  • Yan-Wu He (29/04/18 06:50 PM IDT)
  • Andreas Stiller (29/04/18 09:36 PM IDT)
  • Bart De Vylder (30/04/18 09:57 AM IDT)
  • Muralidhar Seshadri (30/04/18 10:25 AM IDT)
  • Walter Sebastian Gisler (30/04/18 05:54 PM IDT)
  • Hugues Juille (30/04/18 07:08 PM IDT)
  • Erik Hostens (30/04/18 10:50 PM IDT)
  • Shirish Chinchalkar (01/05/18 01:03 AM IDT)
  • Li Li (01/05/18 09:34 AM IDT)
  • Clive Tong (01/05/18 07:15 PM IDT)
  • David Greer (01/05/18 11:44 PM IDT)
  • Graham Hemsley (03/05/18 12:36 PM IDT)
  • Alexander Raß (03/05/18 03:55 PM IDT)
  • Kang Jin Cho (03/05/18 07:33 PM IDT)
  • Loukas Sidiropoulos (03/05/18 09:09 PM IDT)
  • Susanne Wienand (03/05/18 09:43 PM IDT)
  • Dominik Reichl (04/05/18 04:07 PM IDT)
  • Andreas Eisele (04/05/18 04:21 PM IDT)
  • Vladimir Volevich (04/05/18 05:16 PM IDT)
  • Guillaume Escamocher (04/05/18 06:45 PM IDT)
  • Rickard Björklund (05/05/18 05:58 AM IDT)
  • Aviv Nisgav (06/05/18 11:12 AM IDT)
  • Shouky Dan & Tamir Ganor (07/05/18 09:01 AM IDT)
  • Peter Gerritson (07/05/18 11:26 PM IDT)
  • David F.H. Dunkley (08/05/18 01:12 AM IDT)
  • Franciraldo Cavalcante (08/05/18 06:51 PM IDT)
  • Nir Lavee & Omer Shechter (08/05/18 10:05 PM IDT)
  • Francis Golding (08/05/18 11:19 PM IDT)
  • Liubing Yu (09/05/18 08:55 PM IDT)
  • Paolo Farinelli (09/05/18 09:07 PM IDT)
  • Daniel Chong Jyh Tar (10/05/18 04:57 AM IDT)
  • Lorenz Reichel (10/05/18 14:53 IDT)
  • Hugo Pfoertner (10/05/18 08:17 PM IDT)
  • Dieter Beckerle (11/05/18 10:05 AM IDT)
  • Davin Choo & Daren Khu (11/05/18 07:33 PM IDT)
  • Daniel Bitin (11/05/18 09:54 PM IDT)
  • Vladimir M. Milovanović (12/05/18 12:24 AM IDT)
  • Mykola Zubach (12/05/18 12:53 PM IDT)
  • Pål Hermunn Johansen (13/05/18 12:19 PM IDT)
  • Joaquim Neves Carrapa (14/05/18 02:27 PM IDT)
  • Jérémie Bernier (19/05/18 12:25 PM IDT)
  • Steven Langerwerf (19/05/18 06:37 PM IDT)
  • Almog Yair (20/05/18 03:14 AM IDT)
  • Roland Postle (21/05/18 04:19 PM IDT)
  • Andrei Tsygankov (21/05/18 09:57 PM IDT)
  • Parth Trivedi & Dhruvi Unadkat (22/05/18 09:38 PM IDT)
  • Hongcheng Zhu (25/05/18 09:59 AM IDT)
  • Tom Sirgedas (26/05/18 12:42 AM IDT)
  • Nyles Heise (27/05/18 08:42 AM IDT)
  • Mathias Schenker (28/05/18 09:41 AM IDT)
  • Harald Bögeholz (29/05/18 07:25 AM IDT)
  • Motty Porat (29/05/18 06:58 PM IDT)
  • Tyler Mullen (31/05/18 06:35 AM IDT)
  • Radu-Alexandru Todor (01/06/18 02:58 AM IDT)
  • Oscar Volpatti (01/06/18 03:17 PM IDT)

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