What is the general stance on riddles being asked on math.stackexchange.com?

My friends and I are debating a riddle but none of us are sure of what the correct way is to approach it.

If math.stackexchange.com isn't the place to post this, where then?


The riddle:

There are 100 soldiers. 85 lose a left leg, 80 lose a right leg, 75 lose a left arm, 70 lose a right arm. What is the minimum number of soldiers losing all 4 limbs?

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  • $\begingroup$ It depends on whether the riddle is sufficiently mathematical. You might as well post it in this meta question and see what people think. $\endgroup$ – Dylan Moreland Jan 26 '12 at 13:16
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    $\begingroup$ @DylanMoreland I think the riddle would qualify as being mathematical enough. Please see edit. $\endgroup$ – Pieter van Niekerk Jan 26 '12 at 13:19
  • $\begingroup$ This looks okay to me (as WW says, it's best if you post your own thoughts along with the question), if a little macabre! $\endgroup$ – Dylan Moreland Jan 26 '12 at 13:41
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    $\begingroup$ I think that question is fine (I wouldn't really call it a riddle, even). You may want to elaborate, when you post, on how you and your friend reach your differing conclusions. Also, you probably should tag it (pigeonhole-principle) and/or (inclusion-exclusion) $\endgroup$ – Willie Wong Jan 26 '12 at 13:41
  • $\begingroup$ Relevant: math.stackexchange.com/questions/102598/100-soldiers-riddle $\endgroup$ – Eric Naslund Jan 26 '12 at 14:16
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    $\begingroup$ Also this question from meta is perhaps interesting here: PUZZLING — Area 51 proposal $\endgroup$ – Martin Sleziak Jan 26 '12 at 15:17

The purpose of the Math Stack Exchange is as a Question and Answer forum for mathematics.

So in regards to riddles, I think most users would agree with the following tests:

  1. Is the nature of the riddle mathematical? A logic puzzle like knight and knaves is, a clever word play on mathematical terminology ("What's yellow and implies the axiom of choice?") is not.
  2. Do you already know the answer to the riddle? Math.SE is not a "riddle-me-this" forum, and we discourage the asking of a question (or in this case, a riddle) for the sake of "asking a question". (For example, if your motivation is that "this is a fun riddle and people should see it" or "I want to ask a riddle and judge whether people answered correctly" then perhaps this website is not the right forum.)
  3. For logic puzzles, have you done your homework? A lot of logic puzzles have long-standing known solutions easily found on the internet. In some cases the solutions have been discussed on this site before.

In your case, a poor way to ask the question would be:

Here's a riddle. What's the solution?

A slightly better way to ask the question would be:

Here's a riddle. Starting from so-and-so we can deduce thus-and-thus that the answer must be such-and-such. But starting from this-and-that we can deduce here-and-there so the answer must be not-such-nor-such. Which is the right answer?

Though most likely in this case it is just that the hypothesis of the riddle contains two contradictory statements.

BTW, since you already asked this question, Dylan's comment is very practical. But in general there is no need to really vet your question before posting it. Just remember that

  • A question being closed does not reflect poorly on you as a person; unless of course you refuse to learn from the experience and keep posting similar questions over and over again (which, by the way, would be considered spamming).
  • A question being closed does not necessarily reflect that the question, in the abstract, is a bad question: it may just be that the community deems the question unsuitable for this particular forum. (In other words: it's not you, it's us.)

If you can keep that in mind, there's usually no harm to just asking the question on the main site.

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    $\begingroup$ Zorn's lemon? I actually prefer "What can carry you in the Andes and implies the axiom of choice?". $\endgroup$ – Asaf Karagila Jan 26 '12 at 13:47
  • $\begingroup$ @Asaf: or "what do you call a Tibetan monk enslaved by Magneto in disguise, who also implies the axiom of choice?" The lemon version is shorter and sweeter. $\endgroup$ – Willie Wong Jan 26 '12 at 14:12
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    $\begingroup$ "We discourage the asking of a question (or in this case, a riddle) when you already know the answer." That is not universal. E.g. asking and answering one's own question is sometimes encouraged, or asking to learn of other ways of answering. I think as long as the question makes the intent clear (and the question is on topic), this usually works. Maybe it applies in the case of a riddle where there is a unique trick to getting the riddle, but if it is mathematical rather than word play that seems unlikely to happen. $\endgroup$ – Jonas Meyer Jan 26 '12 at 15:26
  • $\begingroup$ @Jonas: I see I didn't make it clear. Let me try to re-write that sentence. $\endgroup$ – Willie Wong Jan 26 '12 at 15:42
  • $\begingroup$ @Jonas: BTW, if the "question" is "how can one solve this problem differently", I would say that the OP does not know the answer already. Like you said, the point of the second bullet is one of intent. $\endgroup$ – Willie Wong Jan 26 '12 at 15:50
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    $\begingroup$ @Willie: Thank you for clarifying. I agree. $\endgroup$ – Jonas Meyer Jan 26 '12 at 15:52
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    $\begingroup$ Something related to @Jonas's comment: In a few cases, the poster has an idea of how to solve the problem and just wants a confirmation that the proof is correct/rigorous etc. This is also usually considered on-topic. [Of course, as Willie points out, here again the "question" is "Is my work correct?" and not "Give me a proof/hint", so the OP does not know the answer already.] $\endgroup$ – Srivatsan Jan 26 '12 at 16:08
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    $\begingroup$ @Srivatsan: I agree. I think "is my work correct" is perfectly on-topic. At least until MSE grows large enough to warrant our own CodeReview like spin-off :p (in which case it may be only off-topic by virtue of having a better site for the question; but let us not get ahead of ourselves here). On the other hand, I don't really know how to answer those questions. I mean, we can't really just post the one word "Yes"... $\endgroup$ – Willie Wong Jan 26 '12 at 19:32
  • $\begingroup$ Willie: Hm, ProofReview.SE is an amusing thought to me right now :-), but yes, it might become necessary and useful in the future. $\endgroup$ – Srivatsan Jan 26 '12 at 19:37
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    $\begingroup$ Willie, it's the lemon flower that is sweet. The fruit of the poor lemon is impossible to eat. Will Holt had nothing to say with regard to the lemon version. mudcat.org/@displaysong.cfm?SongID=3546 $\endgroup$ – Gerry Myerson Jan 26 '12 at 23:59
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    $\begingroup$ @Gerry: have you actually tasted llama? (BTW, nor have I...) I certainly did not claim lemon is a maximal element in terms of sweetness. I only claim that it is not a minimal element. ;p $\endgroup$ – Willie Wong Jan 27 '12 at 8:51

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