[FAQ] rec.puzzles Frequently Asked Questions [weekly]
Jonathan S. Haas
Archive-name: puzzles/faq
Posting-frequency: weekly
Rec-puzzles-archive-name: puzzles/faq
Last-modified: Mon Jul 08 1996
Version: 1.327
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 and to
introduce new readers of rec.puzzles to newsgroup etiquette. Questions
and comments about this FAQ should be emailed to
jh...@microsoft.com
Changes from the last version are marked with a + in the first
column for new lines and a | in the first column for changed lines.
This FAQ is maintained by Jonathan S. Haas, jh...@microsoft.com
It is posted weekly, on Monday.
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.
2.A. What is unusual about this paragraph?
2.B. What are the three common English words that end in -GRY?
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. Posting puzzle solutions.
5. Information about the rec.puzzles archive.
5.1. The archive index.
5.2. Accessing individual puzzles.
5.3. Getting to the archive by FTP, gopher, and WWW.
6. Information about the rec.puzzles oracle.
7. 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 5 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 6 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 5.3 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
11 P in TS of the FAQ [11 puzzles in this section of the FAQ]
NOTE: Tim Vaughan hit the nail right on the head when he said:
1,000,000 N of T these P have been P to R.P
By the way, credit for this puzzle in its original form is due
to Will Shortz of GAMES magazine.
2.A. What is unusual about this paragraph?
It doesn't contain the letter "e". This one has recently been
rearing its ugly head an awful lot lately on rec.puzzles.
Curiously enough, most of the people who post it say something
along the lines of "this one is really super difficult, and
I'll be very surprised if someone gets it". Actually, it's
obvious to most people at first glance.
2.B. ==> gry <== [language/part2]
What are the three common English words that end in -GRY?
There are only two... "angry" and "hungry". The rec.puzzles
archive lists a large collection of words that end in -GRY,
but none of them could be considered even remotely common.
This puzzle began to plague rec.puzzles and other newsgroups in late
March 1996, when a new version appeared with embellishments such as
"you use it every day, and if you've listened closely, I've already
told you the answer." There is evidence that all these versions
originated in an old trick-question, spoken variant of the puzzle
where the listener had to figure out that there were quotation marks
and an ambiguous pronoun reference: the expression "only three words
in 'the English language'", then leads to the answer "language" and
not a word ending in -GRY at all.
A person who doesn't know the trick and asks someone else to try the
puzzle will almost certainly change the wording, accidentally making
it insoluble, and in this case, one of these changed versions has
apparently made it into circulation on radio phone-in shows as well
as the Net.
Either that, or the embellishments were added by a sadist who wanted
to make people rack their brains for a nonexistent word.
Recently, the word "nugry" was coined to provide an answer to this
puzzle. It was first used by Tom Maciukenas (to...@dvorak.amd.com)
and its current definition is:
nugry (noo-gree) n. 1. A newcomer who fails to follow established rules
or procedures. 2. One who shows his inexperience by acting
inappropriately. 3. One who posts the -GRY puzzle to rec.puzzles, in
violation of the FAQ. --adj., Having the qualities of a nugry.
--nugrily, adv., pl. nusgry (news-gree)
----------------------------------------------------------------------
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.
In spring 1994 Sloane announced a nifty new service, the On-Line
Encyclopedia of Integer Sequences. To look up a sequence, send
e-mail to sequ...@research.att.com containing up to five requests
like the following:
lookup 4 9 16 25 36
The reply will report all sequences in the encyclopedia (up to a limit
of seven) that match your sequence. An empty message to that address
will get you more detailed instructions. For further information,
send email to nj...@research.att.com.
3.4. English language records
What's the shortest sentence with all 26 letters? ==> pangram <==
What's the longest one-syllable word? ==> syllable <==
(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".
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. Posting puzzle solutions
When someone posts a puzzle that you know the solution to, or better
yet, posts a puzzle that you discover the solution to after hours
of thinking, it's natural to want to post the solution to that
puzzle to proclaim your achievement to the world. Such posts
are called "spoilers". There's nothing wrong with posting a spoiler,
however, certain etiquette guidelines should be followed.
First, check to see if anyone else has already posted the solution.
There's little point in spoiling a puzzle that's already been spoiled.
Multiple spoilers for a puzzle just eat bandwidth and lower
the signal-to-noise ratio.
Once you've determined that your spoiler won't be redundant, you
can go ahead and compose your followup. It is *important* that
you *warn* people that you are posting a spoiler, so they can
skip your post if they choose. After all, it's no fun to come
across an original and challenging puzzle, look forward to solving
it, and then accidentally read a solution posted by an inconsiderate
poster.
When you post a spoiler, you should include the warning [SPOILER]
after you quote the puzzle, but before you include your solution.
(Several people also include [SPOILER] in the subject line of
their posting.) After the [SPOILER] flag, insert a page break
by pressing control-L; it should look like this: ^L. The Page
break will cause most newsreaders to pause at that point and
give the reader a chance to move on to the next post if they'd
rather not read your spoiler. If your editor won't let you
insert a page break, 20 or 30 blank lines will do.
For people who *do* want to read your spoiler, it's a good
idea to show your work and any reasoning you did. When someone
chooses to read a spoiler, it's usually because that person
has given up on solving the puzzle on his own, and wants to
know how to solve it. A bare answer does no good, and isn't
very entertaining.
[insert examples of "good" and "bad" spoilers here]
----------------------------------------------------------------------
5. 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
(ch...@questrel.questrel.com), 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
archive...@questrel.com. Discussion of the solutions in the
archive is usually welcomed in rec.puzzles.
5.1. 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
archive...@questrel.com, with a body that looks like this:
return_address your_name@your_site.your_domain
send index
For example, if your net address is "mic...@disneyland.com", send this
message:
return_address mic...@disneyland.com
send index
The index will be sent by an auto-reply daemon that runs overnight, so
don't expect an immediate response.
5.2. 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.
5.3. Getting to the archive by FTP, gopher, or WWW
FTP
The entire archive is also accessible via anonymous FTP, from any site
which maintains archives of the newsgroups news.answers or
rec.answers. The file part01 contains the index. The remaining files
contain alternating problem text and solution text for all the
puzzles.
Some FTP sites are:
North America:
ftp://rtfm.mit.edu/pub/usenet/news.answers/puzzles/archive
ftp://ftp.uu.net/usenet/news.answers/puzzles/archive
ftp://mirrors.aol.com/pub/rtfm/usenet/news.answers/puzzles/archive
ftp://ftp.cis.ksu.edu/pub/mirrors/news.answers/puzzles/archive
Europe:
ftp://ftp.cs.ruu.nl/pub/NEWS.ANSWERS/puzzles/archive
ftp://ftp.sunet.se/pub/usenet/news.answers/puzzles/archive
Asia:
ftp://ftp.edu.tw/USENET/FAQ/news/answers/puzzles/archive
GOPHER
From the global home page, the menu choices to access the archives
at "cs.ttu.edu" are:
North America/USA/Texas/Texas Tech University, Computer Sciences
/Entertainment/Games/Puzzles
To access "uni-hohenheim.de" your menu choices are:
Europe/Germany/University of Hohenheim/Lots of Interesting Stuff
/FAQ Frequently Asked Questions/rec/puzzles/archive
WAIS
wais://xraysgi.ims.uconn.edu:8000/rpa
WEB
http://xraysgi.ims.uconn.edu/searchform.html
By keyword as well as subject.
http://einstein.et.tudelft.nl/~arlet/puzzles/index.html
Partially HTMLized.
http://www.nova.edu/Inter-Links/puzzles.html
http://xraysgi.ims.uconn.edu/others.html
A list of other sites (maintained by David Moews)
6. 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:
Comments and suggestions are always welcome. Send them to the oracle,
or to Chris Cole (address given earlier).
----------------------------------------------------------------------
7. CREDIT WHERE CREDIT IS DUE
This FAQ was authored by Tom Magliery, m...@ncsa.uiuc.edu. Many,
many thanks to him for all his hard work putting it together.
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,
Neil Sloane, Tim Vaughan.
--
__/\__ Jonathan S. Haas | Jake liked his women the way he liked
\ / jh...@microsoft.com | his kiwi fruit: sweet yet tart, firm-
/_ _\ Gore in '97 | fleshed yet yielding to the touch, and
\/ Printed on recycled electrons| covered with short brown fuzzy hair.
Jonathan S. Haas
Mark Schnitzius
I think there might be a problem with the FAQ, or at least the
summary that's posted here. In particular:
In article <5g1eef$l...@Starbase.NeoSoft.COM>,
jh...@microsoft.com (Jonathan S. Haas) wrote:
> 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?
There's a subtlety to this problem that's often missed. The way it's
stated here, the answer is 50%, because you are volunteering the
information. If you choose one of your children at random (or by whatever
means you want; oldest, ugliest, by coin flip, etc.) and announce "At
least one of my children is a [whatever]", you have reduced the problem
to 50/50.
The actual FAQ entry that you get by retrieving the oldest.girl problem
has it correct:
----
You meet a stranger on the street, and ask how many children he has. He
truthfully says two. You ask "Is the older one a girl?" He truthfully
says yes. What is the probability that both children are girls? What
would the probability be if your second question had been "Is at least
one of them a girl?", with the other conditions unchanged?
==> probability/oldest.girl.s <==
There are four possibilities:
Oldest child Youngest child
1. Girl Girl
2. Girl Boy
3. Boy Girl
4. Boy Boy
If the stranger says "My oldest child is a girl," he has eliminated
cases 3 and 4, and in the remaining cases both are girls 1/2 of the
time. If the stranger says "At least one of my children is a girl," he
has eliminated case 4 only, and in the remaining cases both are girls
1/3 of the time.
----
Here, the information isn't volunteered until it is asked for, so
the possibility that the answer to the original question might be
"no" must be factored in. A subtle point, but it makes the difference
between a 50% and a 66.666...% answer.
--Mark
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
mark schnitzius isx corporation schn...@east.isx.com
<A HREF="http://east.isx.com/~schnitzi/">me</A>
Practice random kindness and senseless acts of extreme violence.
-------------------==== Posted via Deja News ====-----------------------
http://www.dejanews.com/ Search, Read, Post to Usenet
Jonathan Haas
Mark Schnitzius <schn...@east.isx.com> wrote:
>I think there might be a problem with the FAQ, or at least the
>summary that's posted here. In particular:
>
Absolutely correct. I'll fix it before the next posting.
By the way, Mark, I stand in sincere awe of your (and others') entries
into the Obfuscated C contest. Wish I could code like that... er,
deliberately.
Rob Falconer
In article <8581927...@dejanews.com>, Mark Schnitzius
<schn...@east.isx.com> writes
<snip>
>----
>==> probability/oldest.girl.s <==
>There are four possibilities:
>
> Oldest child Youngest child
>1. Girl Girl
>2. Girl Boy
>3. Boy Girl
>4. Boy Boy
>
>If the stranger says "My oldest child is a girl," he has eliminated
>cases 3 and 4, and in the remaining cases both are girls 1/2 of the
>time. If the stranger says "At least one of my children is a girl," he
>has eliminated case 4 only, and in the remaining cases both are girls
>1/3 of the time.
>----
>
Possible offspring:
#1 Zero girls
#2 One girl
#3 Two girls
If the stranger says 'at least one of my children is a girl' he has
eliminated only #1. In the remaining cases both are girls 1/2 of the
time.
--
Rob Falconer
R...@rfalconr.demon.co.uk
Mathew Hendry
Rob Falconer <R...@rfalconr.demon.co.uk> wrote:
: In article <8581927...@dejanews.com>, Mark Schnitzius
: >==> probability/oldest.girl.s <==
: >There are four possibilities:
: >
: > Oldest child Youngest child
: >1. Girl Girl
: >2. Girl Boy
: >3. Boy Girl
: >4. Boy Boy
: >
: >If the stranger says "My oldest child is a girl," he has eliminated
: >time. If the stranger says "At least one of my children is a girl," he
: >has eliminated case 4 only, and in the remaining cases both are girls
: >1/3 of the time.
: >----
:
: Possible offspring:
: #1 Zero girls
: #2 One girl
: #3 Two girls
: eliminated only #1. In the remaining cases both are girls 1/2 of the
: time.
Nope. #2 is twice as likely as #3.
-- Mat.
Wei-Hwa Huang
posi...@Starbase.NeoSoft.COM (Jonathan Haas) writes:
>By the way, Mark, I stand in sincere awe of your (and others') entries
>into the Obfuscated C contest. Wish I could code like that... er,
>deliberately.
I actually tend to write obfuscated code myself while not particularly
meaning to. I don't deliberately try to mislead any readers, but most
of my optimized-brute-force puzzle solvers use multi-dimensional
nested recursion that can get rather hard to read.
I'll try to dig out the program that I used to solve this and
other similar puzzles:
ObPuzzle: How many ways, not counting trivial transformations,
are there of labeling the faces of 4 octahedral dice so that the
probability distribution of the sum of a random throw of all
four dice is identical to that of 4 octahedral dice with the
orthodox 1..8 labeling?
--
Wei-Hwa Huang, whu...@ugcs.caltech.edu, http://www.ugcs.caltech.edu/~whuang/
-------------------------------------------------------------------------------
Why are they called "digital" watches? They're obviously carpal, not digital.
Rob Falconer
In article <19970316.4...@al056.du.pipex.com>, Mathew Hendry
<sca...@dial.pipex.com> writes
If you classify 'sibling-pairs' into 3 groups-
1 Both are boys
2 One boy / one girl
3 Both girls
Then indeed, as you have indicated,the number of *pairs* in group 2 will
be double the number of pairs in the other two groups,
BUT the number of *girls* in group 2 will *equal* the number of girls in
group 3.
SO Any girl has an equal chance of being in group 1 or group 2. So
given 'a girl from a sibling pair', the probability that her sibling is
also a girl is 0.5
--
Rob Falconer
R...@rfalconr.demon.co.uk
Mike Naylor
> o READ THE NEWSGROUP FOR AT LEAST A MONTH!
If we want people to take the FAQ seriously, we need to remove this absurd
requirement. Also, I think the FAQ needs to be revised to have a less
hostile and arrogant attitude toward newcomers. Remember, the people who
irritate us the most are those who don't read the FAQ. That's no reason for
us to abuse those who do read it.
-
Mike Naylor - PGP key 0x49F92C49
Email myfirstname...@mail.serve.com
Play Five by Five Poker at http://www.serve.com/games
Matthew Daly
In article <332c2264...@nntp.ix.netcom.com> demons...@inscrutable.lucidity (Mike Naylor) writes:
>jh...@microsoft.com (Jonathan S. Haas) wrote:
>> o READ THE NEWSGROUP FOR AT LEAST A MONTH!
>
>requirement. Also, I think the FAQ needs to be revised to have a less
>hostile and arrogant attitude toward newcomers. Remember, the people who
>irritate us the most are those who don't read the FAQ. That's no reason for
>us to abuse those who do read it.
These are good points. And these rules were written when the newsfeed
was nowhere near as good as it is now, and there are new and powerful
technologies at hand.
I would suggest two things:
- Exhorting new users with a specific question to search Deja News
for the question they have, to see if an answer has been posted
in those archives. If so, then there is no sense to bother us
with it again, eh?
- There is a certain use to reading a selection of posts before
writing a post of your own -- you won't be familiar with the
rec.puzzles style of giving humorous answers when someone is
overly general with their question. But a month is so ludicrous
that no one would ever do it. I think that two weeks is more
usual (and you can count a week of it if you exhaust the spool
on your server on your first day).
-Matthew
--
Matthew Daly I feel that if a person has problems communicating
mwd...@kodak.com the very least he can do is to shut up - Tom Lehrer
My opinions are not necessarily those of my employer, of course.
birdman
How about posting rules and regs at the top of the list and describe the
process i.e. how to submit. How to reply. How to "broadcast" answers.
How to congratulate winners. Be generous.
bir...@nbnet.nb.ca
Mike Naylor <demons...@inscrutable.lucidity> wrote in article
<332c2264...@nntp.ix.netcom.com>...
> jh...@microsoft.com (Jonathan S. Haas) wrote:
> >1. WHAT YOU SHOULD DO BEFORE YOU POST A PUZZLE:
> ....
> > o READ THE NEWSGROUP FOR AT LEAST A MONTH!
>
> If we want people to take the FAQ seriously, we need to remove this
absurd
> requirement. Also, I think the FAQ needs to be revised to have a less
> hostile and arrogant attitude toward newcomers. Remember, the people who
> irritate us the most are those who don't read the FAQ. That's no reason
for
> us to abuse those who do read it.
Mark Schnitzius
In article <MDphjBAh...@rfalconr.demon.co.uk>,
Rob Falconer <R...@rfalconr.demon.co.uk> wrote:
> >: >If the stranger says "My oldest child is a girl," he has eliminated
> >: >cases 3 and 4, and in the remaining cases both are girls 1/2 of the
> >: >time. If the stranger says "At least one of my children is a girl," he
> >: >has eliminated case 4 only, and in the remaining cases both are girls
> >: >1/3 of the time.
> >:
> >: #1 Zero girls
> >: #2 One girl
> >: #3 Two girls
> >: If the stranger says 'at least one of my children is a girl' he has
> >: eliminated only #1. In the remaining cases both are girls 1/2 of the
> >: time.
> >
> >Nope. #2 is twice as likely as #3.
>
> If you classify 'sibling-pairs' into 3 groups-
> 1 Both are boys
> 2 One boy / one girl
> 3 Both girls
>
> Then indeed, as you have indicated,the number of *pairs* in group 2 will
> be double the number of pairs in the other two groups,
>
> BUT the number of *girls* in group 2 will *equal* the number of girls in
> group 3.
>
> SO Any girl has an equal chance of being in group 1 or group 2. So
> given 'a girl from a sibling pair', the probability that her sibling is
> also a girl is 0.5
You two are talking about two different problems, which was the point of
my original request to fix the FAQ.
In pseudocode, this is the problem for which you can only predict with 0.5
accuracy:
person 1 has two children of random sex
person 1 picks one of these children at random, and announces
"one of my children is a [whatever]"
Person 2 can only predict with 50% accuracy the sex of the other child.
Now, the other problem goes like this:
person 1 has two children of random sex
person 2 picks a gender, say male, and asks person 1 "is your
child male?"
IF PERSON 1 SAYS YES, person 2 can then guess the sex of the
other child to be female with an accuracy of 2/3.
This works vice versa as well.
Rob, I believe you are talking about the first case. The FAQ, I'm sure,
means to talk about the second case because it is the more surprising
result.
Jonathan S. Haas
Archive-name: puzzles/faq
Posting-frequency: weekly
Rec-puzzles-archive-name: puzzles/faq
Version: 1.328
[note: this edition of the FAQ is a day late, due to problems with
my ISP. I apologize for any inconvenience. If anyone out there is
looking for an ISP, I heartily recommend against Neosoft of Houston]
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 and to
introduce new readers of rec.puzzles to newsgroup etiquette. Questions
and comments about this FAQ should be emailed to
jh...@microsoft.com
This FAQ is maintained by Jonathan S. Haas, jh...@microsoft.com
0. INTRODUCTION AND CONTENTS
----------------------------------------------------------------------
1. WHAT YOU SHOULD DO BEFORE YOU POST A PUZZLE:
Before you post any puzzle to rec.puzzles, you should take the
----------------------------------------------------------------------
2.5. ==> oldest.girl <== [probability]
If a person has two children, and answers "yes" to the question
"Is at least one of your children a girl?", what is the
probability that both children are girls?
+---+---+---+
| | | |
+---+-+-+---+
| | |
+-----+-----+
----------------------------------------------------------------------
3.3. Sequence puzzles
3.4. English language records
3.5. Logic puzzles
----------------------------------------------------------------------
4. Posting puzzle solutions
5.1. The archive index
return_address your_name@your_site.your_domain
send index
return_address mic...@disneyland.com
send index
5.2. Accessing individual puzzles
send <puzzle-name>
send logic/weighing/balance
send balance
FTP
Some FTP sites are:
North America:
Europe:
ftp://ftp.cs.ruu.nl/pub/NEWS.ANSWERS/puzzles/archive
ftp://ftp.sunet.se/pub/usenet/news.answers/puzzles/archive
Asia:
ftp://ftp.edu.tw/USENET/FAQ/news/answers/puzzles/archive
GOPHER
WAIS
wais://xraysgi.ims.uconn.edu:8000/rpa
WEB
6. THE rec.puzzles ORACLE
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--