May 2018 - Challenge

<< April


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.


Update 1/5:

  1. The two counterfeit coins have exactly the same weight.
  2. You must provide all the weighings in advance; you can't change any of them based on the results of previous weighings.


We will post the names of those who submit a correct, original solution! If you don't want your name posted then please include such a statement in your submission!

We invite visitors to our website to submit an elegant solution. Send your submission to the ponder@il.ibm.com.

If you have any problems you think we might enjoy, please send them in. All replies should be sent to: ponder@il.ibm.com

Challenge: 26/04/2018 @ 12:00 PM EST
Solution: 02/06/2018 @ 12:00 PM EST
List Updated: 23/05/2018 @ 12:00 PM EST

People who answered correctly:

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 (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)