play and relax: games for kids games
  Cut the knot: learn to enjoy mathematics
A math books store at a unique math study site. Learn to enjoy mathematics.
Google
Web CTK
Best sites for teachers
Sites for teachers
Sites for parents
Terms of use
Awards

Interactive Activities
CTK Exchange
CTK Insights - a blog

Games & Puzzles
What Is What
Arithmetic/Algebra
Geometry
Probability
Outline Mathematics
Make an Identity
Book Reviews
Eye Opener
Analog Gadgets
Inventor's Paradox
Did you know?...
Proofs
Math as Language
Things Impossible
Visual Illusions
My Logo
Math Poll
Cut The Knot!
MSET99 Talk
Other Math sites
Front Page
Movie shortcuts
Personal info
Privacy Policy

Guest book
News sites

Recommend this site

Best sites for teachers
Sites for teachers
Sites for parents

Education & Parenting

Manifesto: what CTK is about Search CTK Buying a book is a commitment to learning Table of content Things you can find on CTK Chronology of updates Email to Cut The Knot Recommend this page

Looping Chips Puzzle

In this puzzle, 12 squares are arranged in a chain pattern with four chips of different colors covering the first four squares. You are allowed to drag a chip to a square 5 places from its current location, either clockwise or counterclockwise. The purpose of the puzzle is to have the chips in the originally occupied four squares but in the reverse order, or to determine that this is not possible.

You always play on the left board. The right one is very helpful and it may be worth your while to figure out its relationship to the left board.

 

This applet requires Sun's Java VM 2 which your browser may perceive as a popup. Which it is not. If you want to see the applet work, visit Sun's website at http://www.java.com/en/download/index.jsp, download and install Java VM and enjoy the applet.


Buy this applet
What if applet does not run?

Explanation

Copyright © 1996-2008 Alexander Bogomolny

 

 

 

 

 

 

 

 

 

 

Explanation

The same logic that worked for 4 Knights puzzle works here as well. From every square their are only two possible moves. If we count squares according to a sequence of eligible moves then the right board represents these moves sequentially. On the right board the chips can't jump over each other. Therefore, their order on the right is unchangeable. The same is true with regard to any four fixed squares on the left.

The puzzle suggests several variations. There are three numeric parameters:

  1. N, the number of squares.
  2. M, the number of chips.
  3. L, the length of a move.

Above, N = 12, M = 4, L = 5. The number of chips M is only important to limit L from below. For example, with M = 4 the least meaningful move should unable to skip over 3 occupied squares and onto the next one. So L could not be less than M. (The size of the puzzle, N obviously imposes an upper bound as well.) When N and L are coprime any chip may eventually reach any square. If gcd(N,L) > 1 the board on the right will have to be split into separate and non-intersecting loops. For example, for N = 12 and L = 4, there will be four loops:

  1 > 5 > 9 > 1,
2 > 6 > 10 > 2,
3 > 7 > 11 > 3,
and 4 > 8 > 12 > 4

which makes it even more obvious that the chips can't swap places.

It follows then that however we change the three parameters of the puzzle, the puzzle is, in principle, insolvable. Perhaps, we can change the basic rules? Right now, the set of eligible moves depends on the location of a square from where a move is executed. What if we make the set of eligible moves depend on the chip that makes a move. Say, chips 1 and 2 are allowed to skip 4 places whereas chips 3 and 4 skip 5 places as before. Will this change anything?

Copyright © 1996-2008 Alexander Bogomolny

29436676Page copy protected against web site content infringement by Copyscape


Search:
Keywords:


Latest on CTK Exchange
try this puzzle ?/?? + ?/?? + ?/? ...
Posted by albert1950
0 messages
07:05 AM, Jul-10-08

Monty Hall Problem
Posted by linkdon
72 messages
06:07 PM, Jul-24-08

Missing information
Posted by roboknight
2 messages
07:32 AM, Jun-22-08

Can You See The Patterns..?
Posted by wustvn
0 messages
10:08 AM, Jul-23-08

Mistake on the page (an aside, Be ...
Posted by Max
4 messages
10:28 AM, Feb-28-08

Central Limit Theorem proof problem
Posted by Manuel
1 messages
01:54 PM, Jul-22-08

You can drill a square hole
Posted by Giorgis
1 messages
10:15 PM, Jul-12-08