Internet Info 1994 March/Internet Info CD-ROM (Walnut Creek) (March 1994).iso/answers/rec/puzzles/faq

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From: [e-mail] (Tom Magliery)
Subject: rec.puzzles Frequently-Asked Questions [monthly]
Message-ID: <[e-mail]>
Followup-To: poster
Summary: This posting contains a list of puzzles that appear more often
         than one would hope on rec.puzzles, and information about the
         rec.puzzles archive, which contains many common and interesting
         puzzles.  Please read both before posting to rec.puzzles.
Originator: [e-mail]
Sender: [e-mail]
Reply-To: [e-mail]
Organization: University of Illinois, Dept. of Comp. Sci., Urbana, IL
Date: Thu, 31 Mar 1994 16:11:29 GMT
Approved: [e-mail]
Expires: Fri, 29 Apr 1994 06:00:00 GMT
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Xref: bloom-beacon.mit.edu rec.puzzles:12817 rec.answers:4688 news.answers:17041

Archive-name: puzzles/faq
Rec-puzzles-archive-name: puzzles/faq
Last-modified: Thu Mar 31 10:10:10 CST 1994
Version: 1.1


Welcome to the rec.puzzles Frequently-Asked Questions List.  The
purpose of this article is to assist readers in determining if their
nifty new puzzle is not quite so nifty or new after all.  Questions
and comments about this FAQ should be emailed to [e-mail].  The
# FAQ was last modified on 31 March 1994.  Changes since the previous
version are marked with '#' in the first column.


0.  INTRODUCTION AND CONTENTS

The rec.puzzles newsgroup is generally a friendly one, and the
signal-to-noise ratio is relatively high compared with that of other
Usenet newsgroups.  However, many rec.puzzles readers have a MAJOR PET
PEEVE -- seeing the same old puzzles (and the same old answers, and
the same old discussions) over and over and over and ...

Please, before you post a puzzle to rec.puzzles, heed the information
in this article!  Briefly, this consists of:

0.  Introduction and contents.
1.  What you should do before you post a puzzle.
2.  Frequently-asked puzzles (specific).
2.1.  Monty Hall.
2.2.  You have 12 coins, one of which is EITHER light or heavy...
2.3.  The bellboy (where is the missing dollar?).
2.4.  1, 11, 21, 1211, ???, ....
2.5.  What's the probability that my other child is a girl?
2.6.  What color is the bear?
2.7.  Two envelopes, one contains twice as much money as the other...
2.8.  Can you draw a line through all the segments in this diagram...
2.9.  26 L of the A.
3.  Frequently-asked puzzles (general).
3.1.  Situation puzzles.
3.2.  Weighing/balance puzzles.
3.3.  Sequence puzzles.
3.4.  English language records.
3.5.  Logic puzzles.
4.  Information about the rec.puzzles archive.
4.1.  The archive index.
4.2.  Accessing individual puzzles.
4.3.  Getting to the archive by FTP.
5.  Information about the rec.puzzles oracle.
6.  Credit where credit is due.

----------------------------------------------------------------------

1.  WHAT YOU SHOULD DO BEFORE YOU POST A PUZZLE:

Before you post any puzzle to rec.puzzles, you should take the
following steps to ensure that the puzzle is not an oldie.

   o  READ THE NEWSGROUP FOR AT LEAST A MONTH!  (Reading for some time
          before posting is proper etiquette for ANY Usenet newsgroup.
          If you did not realize that, it would be a good idea to read
          the advice that's posted regularly to news.announce.newusers.)

   o  Read this article.  (Good for you!  You're doing it!)

   o  Look through the rec.puzzles archive (see Section 4 below).
          If you can't tell whether your puzzle is in the archive
          (it's not always clear from the index), ask the oracle.

   o  Ask the rec.puzzles oracle (see Section 5 below).

If your nifty new puzzle was given to you as an Nth-generation
photocopy, or sent to you by email from someone who got it by email
from someone who..., there's a VERY good chance it's been around in
rec.puzzles before.  Follow the steps above before posting.

If what you really want is not to pose your question for the
puzzlement of rec.puzzles readers, but rather simply to find out the
answer to the puzzle (presumably fairly quickly), then your best bet
is to go straight to the archive, possibly followed by the oracle.

----------------------------------------------------------------------

2.  FREQUENTLY ASKED PUZZLES (SPECIFIC):

This section contains, in no particular order, short summaries of the
most commonly-occurring puzzles on rec.puzzles.  Unless you have some
really devastatingly new twist or observation on these puzzles, you
probably shouldn't post.  Exception:  Discussion of the archive's
solutions to puzzles is often welcomed in the newsgroup.  (But it
might be a good idea to check with the oracle about it first.)

In this section, and throughout the FAQ, the phrases between arrows
==> like this <== are the names of puzzles as they appear in the
archive.  The stuff in [square brackets] is the name of the file you
should look for, once you're in the rec.puzzles archive directory, if
you are accessing the archive by ftp (see Section 4.3 below).


2.1.  ==> monty.hall <==   [decision]
     Three doors; one has a prize.  If you pick the door with the
     prize, you get the prize.  You pick a door, Monty Hall opens
     another door revealing a dud, and offers you the option to switch
     doors.  Should you switch?

     NOTE:  This may be the grand champion of all tiresome puzzles on
     rec.puzzles.  Without a doubt, it has generated more bandwidth
     than any other puzzle over the last few years.  No matter how
     many times we smack it, it keeps rearing its ugly head.  (Not
     unlike Whack-A-Mole in that respect...)


2.2.  ==> balance <==   [logic/part5]
     You have 12 coins, one of which is counterfeit.  The counterfeit
     is indistinguishable from the rest except that it is either heavier
     or lighter (but you don't know which).  How can you determine which
     coin is the counterfeit in 3 weighings on a balance scale?

     NOTE:  This puzzle occasionally gives Monty Hall stiff
     competition for overall tiresomeness, not because of long
     regurgitated discussions, but because at times it seems to be
     re-posted about every other week.


2.3.  ==> 29 <==   [logic/part1]
     Three men in a hotel, each pays $10, the bellboy pockets $2, etc.
     What happened to the missing dollar?


2.4.  ==> series.07 <==   [series]
     What is the next number in this sequence?  1, 11, 21, 1211, ...

     NOTE:  Probably because it appears, without a solution, in Cliff
     Stoll's book _The Cuckoo's Egg_, this has become the grand
     champion of tiresome sequences, the general category of which
     warrants mention in Section 3 below.


2.5.  ==> oldest.girl <==   [probability]
     At least one of my children is a girl.  What is the probability
     that the other one is a girl?


2.6.  ==> bear <==   [geometry/part1]
     A hunter walks south 1 mile, then east 1 mile, then north 1 mile,
     ends up where he started, and shoots a bear.  What color is the
     bear?

     NOTE:  Not only does everyone know the "obvious" answer, but
     everyone knows the infinite number of "less obvious" answers as
     well.  Everyone also knows where polar bears *really* live.


2.7.  ==> envelope <==   [decision]
     I have two envelopes, one of which contains twice as much money
     as the other.  You choose one envelope, I show you the money in
     it, and then give you the choice of keeping it, or taking the
     money in the unopened envelope.  Should you switch?


2.8.  ==> konigsberg <==   [geometry/part1]

     Can you draw a line through each edge on the diagram below
     without crossing any edge twice and without lifting your pencil
     from the paper?

             +---+---+---+
             |   |   |   |
             +---+-+-+---+
             |     |     |
             +-----+-----+


2.9.  ==> equations <==   [language/part1]
     26  L of the A
      8  S on a SS
      9  P in TS of the FAQ   [9 puzzles in this section of the FAQ]

     NOTE:  Credit for this puzzle in its original form is due to
     Will Shortz of GAMES magazine.

----------------------------------------------------------------------

3.  FREQUENTLY ASKED PUZZLES (GENERAL):

This section gives information on some general categories of puzzles
that pop up over and over again.


3.1.  ==> situation.puzzles <==   [logic/part3 and logic/part4]

     A man lies dead with 53 bicycles in front of him.

     A man walks into a bar, asks for a drink, the bartender points a
     gun at him, the man says "thank you" and leaves.

Nobody's sure what these are really called, but here on rec.puzzles we
call them "situation puzzles".  The keeper of the canonical list of
situation puzzles is Jed Hartman.  Comments, questions, suggestions,
etc., should be sent to him.  For information on how to contact him,
read the archive entry.


3.2.  Weighing and balance scale puzzles

The 12-objects puzzle mentioned in the previous section is one example
of this type of puzzle.  There are many variations, and we've seen
most of them in rec.puzzles.  In the archive index, look under the
general category "logic/weighings".


3.3.  Sequence puzzles

     O, T, T, F, F, S, ?               ==> series.06 <==   [series]
     5, 6, 5, 6, 5, 5, 7, 6, ?         ==> series.21 <==   [series]
     ...

The problem with letter sequences is that we've seen most of them,
quite possibly even that one you just thought of all by yourself.
Check out the archive, in the general category "series", to make sure
yours isn't there.

The problem with number sequences is that many puzzlers find them
unsatisfying, since there are infinitely many formulas that will fit
any finite sequence, and the concept of "simplest" or "best" formula
is a slippery one.  Since number sequences inevitably lead to the same
old discussion, it's probably best to avoid them in rec.puzzles.  Fans
of number sequences may enjoy _A Handbook of Integer Sequences_, by
N.J.A. Sloane, Academic Press, New York, 1973, and _Supplement I to A
Handbook of Integer Sequences_, Sloane, Bell Labs, Murray Hill, New
Jersey, 1974.


3.4.  English language records

     What's the shortest sentence with all 26 letters?    ==> pangram <==
     What's the longest one-syllable word?                ==> syllable <==
     What three common English words end in -gry?         ==> gry <==
                                   (all of those are in:  [language/part2])

These and zillions of other questions -- almost anything you could
ever think of -- are answered under the general archive category
"language".  By the way, there are only two *common* English words that
end in -gry, but there are scads of uncommon ones.


3.5.  Logic puzzles

There are a bajillion categories and variations of logic puzzles.
Some familiar ones from the archive are:

==> number <==   [logic/part1]
     Mr. S and Mr. P are "perfect logicians".  Mr. S is given the sum
     of two numbers; Mr. P is given their product.  The following
     conversation ensues.  ...

==> unexpected <==   [logic/part5]
     Swedish civil defense authorities announced that a civil defense
     drill would be held one day the following week, but the actual
     day would be a surprise.  ...

==> ropes <==   [logic/part2]
     Two fifty foot ropes are suspended from a forty foot ceiling,
     about twenty feet apart.  Armed with only a knife, how much of
     the rope can you steal?

In addition to miscellaneous logic puzzles, there are numerous puzzles
of the sort found in books by logician Raymond Smullyan.  Examples of
these are "liar/truthteller" puzzles (wherein the puzzle is to
determine the answer to some problem by posing questions to people,
some or all of whom always lie or never lie) and "unseen mark" puzzles
(wherein a number of people have marks that can be seen only by
others, and the puzzle is to determine how the people can figure out
their own marks).

Before you post a logic puzzle to rec.puzzles, see the archive
category "logic", especially the subcategory "logic/smullyan".  Better
still, read all of Smullyan's books.

----------------------------------------------------------------------

4.  THE rec.puzzles ARCHIVE

Formerly known as the "rec.puzzles FAQ", the archive is a treasure
trove of puzzles and their solutions.  Maintained by Chris Cole
([e-mail]), the archive currently contains over 500
puzzles, including those mentioned in this FAQ, and others of many
varieties.  (It is not, as the name might suggest, an archive of all
postings that come down the pike in rec.puzzles.)

Corrections to and comments on archive entries should be emailed to
[e-mail].  Discussion of the solutions in the
archive is usually welcomed in rec.puzzles.


4.1.  The archive index

The easiest way to figure out the archive is to get and read the
index.  The index contains brief descriptions of all of the puzzles in
the archive.  To request a copy of the index, send email to
[e-mail], with a body that looks like this:

return_address your_name@your_site.your_domain
send index

For example, if your net address is "[e-mail]", send this
message:

return_address [e-mail]
send index

The index will be sent by an auto-reply daemon that runs overnight, so
don't expect an immediate response.


4.2.  Accessing individual puzzles

Individual puzzles from the archive may be requested by sending email
to the same address as above, containing one or more lines of the form

send <puzzle-name>

where <puzzle-name> is the name by which the puzzle is called in the
archive index.  You may use the full name or just the last part of it.
For example, to request the 12-coins problem (see Section 2 above),
either of the following "send" lines will work:

send logic/weighing/balance
send balance

As with the index, an auto-reply daemon will respond overnight.

You may also request multiple puzzles by putting multiple "send" lines
in your request message.  Please refrain from requesting the entire
archive by email.  Use FTP.


4.3.  Getting to the archive by FTP

The entire archive is also accessible via anonymous FTP, from any site
which maintains archives of the newsgroup news.answers.  One such site
is rtfm.mit.edu, where the archive is in the directory
/pub/usenet/news.answers/puzzles/archive.  Because the text of the
archive is so large, it is broken into a number of files with names
according to the types of puzzles in each part.  The file Instructions
contains the index.  The remaining files contain alternating problem
text and solution text for all the puzzles.  Occasionally a single
puzzle is split between files.

Some other news.answers anonymous FTP archives are:

   Site                  IP address        Directory                 Notes
   ---------------------------------------------------------------------
   ftp.cs.ruu.nl         131.211.80.17     /pub/NEWS.ANSWERS         [1]
   ftp.uu.net            137.39.1.9        /usenet/news.answers
                           or 192.48.96.9
   ftp.win.tue.nl        131.155.70.100    /pub/usenet/news.answers
   grasp1.univ-lyon1.fr  134.214.100.25    /pub/faq                  [2]
   ---------------------------------------------------------------------
   [1] Also accessible via mail server requests to [e-mail].
   [2] Also accessible via mail server requests to
       [e-mail].  Best used by EASInet sites.

Note that the periodic posting archives on rtfm.mit.edu are also
accessible via Prospero and WAIS (the database name is "usenet" on
port 210).


5.  THE rec.puzzles ORACLE

This is a group of rec.puzzles regulars, who are familiar with the
rec.puzzles archive, and who will find your answer there if it exists,
or maybe compose an original answer if they are interested enough!
At any rate, they promise to respond to your question within two days,
and perhaps save you the embarrassment of posting a well-worn
question.  They will respond within two days even if they do not know
the answer to your question.

To query the rec.puzzles oracle, send email containing your question
to the following address:

        [e-mail]

Comments and suggestions are always welcome.  Send them to the oracle,
or to Chris Cole (address given earlier).

----------------------------------------------------------------------

6.  CREDIT WHERE CREDIT IS DUE

The following people have proofread, criticized, corrected, or
otherwise contributed to this FAQ:  Mark Brader, Chris Cole, David
Grabiner, Jed Hartman, David Karr, Stein Kulseth, Jerry McCollom.

I thank them all for their suggestions.  To see your name in the above
list, help me find my mistakes!  I also appreciate constructive
comments on content and style.  My email address is:  [e-mail]
--
Tom Magliery ** Dept of CS ** 1304 W Springfield ** Urbana IL 61801 ** USA