## I dare you to solve this.

A forum for good logic/math puzzles.

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apeman5291
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aguacate wrote:What tools were you allowed to use? I'll admit to using a list of the primes and a prime factor program.

anything you might happen to have on hand at a math competition, so you're fine.

H-Bar
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aguacate wrote:
Spoiler:
Part I:
2, 3, 8, 9

Part II:
R = {2893, 3289, 3982, 8239, 9328, 9823}

Spoiler:
I got the same answer for part 1, but for part 2 I also had {..., 2398, 8932} in addition to the ones you listed.

I don't see any reason to exclude them. (Just to make sure we're on the same page I had C,D,H,I false, all others true. My factor for G was 11.) R is the largest valid subset of S, so unless I made a mistake the larger set should be the answer.

There were two things I really liked about this puzzle. First, learning what everything in 9 meant after I looked it up (I solved part 1 first, but I wanted to reassure myself before continuing to part 2). Second, the slap-yourself-in-the-forehead moment when you recognize why E is necessarily true.

By the way, I also used a program for the factorization. Everything else in the problem can be done by hand fairly easily, but doing all those factors manually just sounds like punishment.

davee123
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So, back to the original problem here:
http://www.drunkmenworkhere.org/170

Am I missing something?

Number 12 looks to me like it MUST be (A), but it tells me that's wrong (as though I contradicted my own logic?), when it isn't. At least not yet!

The way I see it, #12 can be 0-20.
But if the answer were "a prime" (for example), then the answer would ALSO be "an odd number". Therefore, I would figure that the answer CANNOT be "a prime", because it would necessarily overlap with the answer "an odd number". Similarly, "a perfect square" and "divisible by 5" also overlap with "an odd number" and "an even number", which therefore means that it CAN'T be prime, CAN'T be divisible by 5, and CAN'T be a perfect square.

Therefore, that means the only viable solutions are A or B. 0,1,4,9, and 16 are out (perfect squares), 5,10,15,20 are out (multiples of 5), 2,3,5,7,11,13,17,19 are out (primes)

Therefore, only options left are 6,8,12,14,18, which are all even, making the answer be (A).

Then, I go ahead and fill in question 15 which is "The answer to question 12 is:", to which I similarly fill in (A). It still marks me correct.

THEN, I go to question #13, which asks which is the only ODD-NUMBERED problem to have answer (A), which turns out to be the question I just answered, #15. But as soon as I do that, it seems to mark me as incorrect? What gives?

Can the answer to 12 overlap with itself? (That would seem dumb). Is it not really telling me I'm wrong? (Horrible interface if so). Am I misreading one of the questions?

DaveE

Mouffles
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When it says number 12 is wrong, it's telling you that at that point your answers are not consistent, becasue you haven't answered an even number of questions with consonants. It doesn't mean the answer is not A.
In the spirit of taking things too far - the 5x5x5x5x5 Rubik's Cube.

une see
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That was pretty tough, I have to admit. Took me about an hour, although I made a huge mistake interpreting question 18. It was pretty fun, though, albeit a little frustrating.
T.S. Eliot in "The Waste Land" wrote:APRIL is the cruellest month, breeding
Lilacs out of the dead land, mixing
Memory and desire, stirring
Dull roots with spring rain.

MFHodge
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davee123 wrote:Can the answer to 12 overlap with itself? (That would seem dumb). Is it not really telling me I'm wrong? (Horrible interface if so). Am I misreading one of the questions?

I had trouble in the same spot. The way that I worked the puzzle is that any true answer to the question could be correct. So if the number is 4, "even" and "perfect square" could both be the answer, but only one will give you a complete solution to the puzzle.

I printed it out and wrote all the applicable numbers next to each letter and crossed them out as I eliminated them. That worked out OK for me.

Minirogue
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VTHodge wrote:I printed it out and wrote all the applicable numbers next to each letter and crossed them out as I eliminated them. That worked out OK for me.

yeah, I numbered a paper from top to bottom 1-20 and wrote ABCDE next to each number and crossed them out. I would think that is the only way unless you are a superultramega-genius, in which case you wouldn't need my advice anyways. Fun quiz though, took me a while but I finally got it with (I suppose) minimal wrong guesses.

shinjak
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it's not necessary. the puzzle is very well constructed and you can work it out in a shortish time (i think i took ~20 mins, and i'm slightly drunk) with no pen, no paper and without keeping too much in your head. you just have to have faith in the puzzle's creator.

it's the question about the barometer that threw me. doesn't a barometer measure atmospheric pressure only?

une see
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I didn't use pen and paper, although in hindsight I probably should have. Um...I didn't really have a problem with question 12. Question 18 was the killer for me, and that spurred on a lot of questions about questions 3, 4, 1, 2, and I think 8. I made an incorrect assumption in the beginning, so that wasted a lot of time for me.
T.S. Eliot in "The Waste Land" wrote:APRIL is the cruellest month, breeding
Lilacs out of the dead land, mixing
Memory and desire, stirring
Dull roots with spring rain.

MFHodge
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(I may be repeating myself but) One thing that really messed me up was when I read the word "consonant" (like not a vowel) as "constant" (like not a variable). I couldn't figure out exactly what it meant, but I was trying to solve it as "the number of questions that have number answers that depend on other questions." That didn't work out for me.

Minirogue
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shinjak wrote:it's the question about the barometer that threw me. doesn't a barometer measure atmospheric pressure only?

I think that is the point. The tests creator must think that standardized tests don't measure intelligence. Kind of a joke of his I think.

Token
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shinjak wrote:it's the question about the barometer that threw me. doesn't a barometer measure atmospheric pressure only?

But note that atmospheric pressure is not one of the answers.

Saru
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Has anyone noticed what the letters spell out when finished?
It is a mistake? something more sinister?
(or has someone pointed this out already?)

jestingrabbit
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Saru wrote:Has anyone noticed what the letters spell out when finished?
It is a mistake? something more sinister?
(or has someone pointed this out already?)

Yes, this guy, on the previous page did, and he believes its a clue to something, so don't read his response unless you don't want to figure the clue out.

http://forums.xkcd.com/viewtopic.php?p=126280#126280

You may or may not be in the sort of age bracket where you'd be able to solve the riddle. If you'd like to tell us about your age bracket, and perhaps a few other tasty tidbits about your life, have a look over at the introduction thread mach III: its three times the goodness.

http://forums.xkcd.com/viewtopic.php?t=10495

ercdvs
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I had to register to post this.. because it drove me nuts.... did this puzzle change answers ?

I 'knew' my answers were correct.. but the puzzle kept marking me as red ... I went so far as to track down the original puzzle and answer key.

I do NOT agree with the question 6/17 pairing... the answer as from the 'real' answer key seems to not adhere to the proper question / answer format...

The answer as it SHOULD be :

Spoiler:

My answer that works out to be correct.. or am I just missing something ?

Spoiler:

The only way I see the solution working as posted is if you treat the proper answer for question '6' as not the a-e answer key, but the actual value for the answer... its against every logic to do so...

jestingrabbit
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I think its right, not you.

Spoiler:
Your answer has 16 and 17 the same, but that contradicts the answer to question 2.

ercdvs
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Ahhhh.... i see that now... but I still maintain that a basic change in the logic of what is a answer vs what value an answer has is needed.

I went into this thinking that the answer to any given question is a,b,c,d or e ... NOT whatever is next to the letter.

jestingrabbit
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ercdvs wrote:Ahhhh.... i see that now... but I still maintain that a basic change in the logic of what is a answer vs what value an answer has is needed.

I went into this thinking that the answer to any given question is a,b,c,d or e ... NOT whatever is next to the letter.

I'm pretty sure that the understanding that the writers of the quiz have is the same as your understanding.

phlip
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shinjak wrote:it's the question about the barometer that threw me. doesn't a barometer measure atmospheric pressure only?

A barometer measures air-pressure, which is reasonably (but not perfectly) correlated with air temp, wind speed, lat and long. Standardised tests measure how good you are at standardised tests, which is reasonably (but not perfectly) correlated with intelligence.

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`enum ಠ_ಠ {°□°╰=1, °Д°╰, ಠ益ಠ╰};void ┻━┻︵​╰(ಠ_ಠ ⚠) {exit((int)⚠);}`
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Ten Thousand Fists
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I can only get the one about the barometer and the 10,16 pair.
where should I look next?

phlip
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Ten Thousand Fists wrote:I can only get the one about the barometer and the 10,16 pair.
where should I look next?

Can question 1 be A?
Can question 1 be B?
Can question 2 be B?
Question 13 you should be able to narrow down to a single answer.

When I did it, I printed it out... so that I could cross out the answers that I definitely knew were wrong... there's a lot more of them at the start than ones that you definitely know are right...

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`enum ಠ_ಠ {°□°╰=1, °Д°╰, ಠ益ಠ╰};void ┻━┻︵​╰(ಠ_ಠ ⚠) {exit((int)⚠);}`
[he/him/his]

hthall
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### Re: I dare you to solve this.

Nice puzzle. I turned off scripts because it was distracting to have things marked inconsistent when I hadn't filled out all the answers yet. It took me a little under an hour (and two square inches of the back of an envelope) to conclude that the answer is unique.

I agree fully with the point made in question 20, but I was a little disappointed that the solution actually depends on this "outside information": if an alternate answer is allowed for question 20, there are four different ways to answer questions 14, 18, 19, and 20 consistently. Of course, only one of these spells a sentence and avoids answer "C"--nice touches, those.

About the matrix puzzle: yes, it's an addition table, with each entry the sum of a hidden row-contribution and column-contribution. The sum of all ten row- and column-contributions is the only score possible.
Look at me, still talking when there's Science to do.

Droooo
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### Re: I dare you to solve this.

I managed to solve the puzzle, but I'm pretty sure my answer doesn't make a sentence, and apparently the solution is unique. Am I missing something here?
Like Drooo but 10x better.

phlip
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### Re: I dare you to solve this.

Droooo wrote:Am I missing something here?

Possibly... post your solution (in [spoiler][/spoiler]) and we'll take a look at it.

Code: Select all

`enum ಠ_ಠ {°□°╰=1, °Д°╰, ಠ益ಠ╰};void ┻━┻︵​╰(ಠ_ಠ ⚠) {exit((int)⚠);}`
[he/him/his]

Poots
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### Re: I dare you to solve this.

So I know this is extremley old and all, but I FINALLY solved it! I started months ago then gave up and managed to solve it relativley quickly, and yea, I'm pretty stoked about that.

JamesCFraser
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### Re: I dare you to solve this.

I hate these. I wrote a quick program to solve one of these a while ago, it had 30 questions. Do people consider that to be cheating? (It took five minutes as opposed to thirty minutes doing it with a program )

freddyfish
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### Re: I dare you to solve this.

i got the puzzle in 50 min!
w/ a pen!
and no writing of notes besides crossing out incorrect answers as i went!

(im a little too proud of myself right now)
its a good puzzle!

Stereo
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Location: Southwest Ontario

### Re: I dare you to solve this.

Serious overthinking on 14, 18 and 19 They were the last 3 to fill in, and if I'd gone with instinct & ignored the whole "wrong answer" marker business it would have been done about 5 minutes sooner.
Clarissa Explains It All, 7 pm (8 Central)

Icewalker
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### Re: I dare you to solve this.

Oooh, I love this puzzle. Finished it with a friend a while back, took us an hour or two split up throughout the school day. Great puzzle. He may have gotten it from here actually, as he introduced me to XKCD.

(First post, other than the one in the greeting thread)

kriel
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### Re: I dare you to solve this.

First of all, somebody needs to change the white text. I can still read it against the blue background. (I think that's what the spoiler tag is for... right? ~.^

Second, well...
Spoiler:
I'm working my way through, and logic games (especially circular ones) have always been a weak point of mine. So, I'm going along... Here's my current answer bank by the way.
1 bcde
2 abcde
3 abcde
4 abcde
5 bcde
6 abcde
7 abcde
8 abcde
9 a
10 a
11 abcde
12 abcde
13 abcde
14 abcde
15 abcde
16 d
17 abcde
18 abcde
19 abcde
20 e

Note 9 and 10. I figured out the 10/16 pair... and then the obvious answer to 9 follows. However, checking the other solutions... no good. However, I can't see any other possible solution for 10/16, and the 9, well... follows. O.o
Where's my logic bomb ticking away at? -.-

EDIT: And now 13's yelling at me for picking A (9) When obviously 9 is an odd-numbered problem with answer A (That' it's not calling out as wrong!) -.-

Owehn
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### Re: I dare you to solve this.

Spoiler:
If your answer to 13 is A, that means you think 9 is the only odd-numbered question with answer A. But then 13 is another odd-numbered question with answer A!

Remember that the form marks an answer wrong when it's inconsistent with the other answers, so while you're filling in the slots it might mark answers wrong that turn out ultimately to be correct (or vice versa).
[This space intentionally left blank.]

kriel
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### Re: I dare you to solve this.

Spoiler:
I misunderstood #9. I thought since 10 was A, 9 had to be A. However, you can choose any of the answers for 9, so long as it comes up true. (ex 10A, 11B, 12C, etc)
EDIT: Current value table.
1 bcde
2 abcde
3 abcde
4 abcde
5 bcde
6 bd
7 abcde
8 abcde
9 abcde
10 a
11 abcde
12 abcde
13 bd
14 abcde
15 abcde
16 d
17 bd
18 abcde
19 abcde
20 e

EDIT2: Get in the spoiler! >.<

TheSwaminator
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### Re:

shinjak wrote:it's the question about the barometer that threw me. doesn't a barometer measure atmospheric pressure only?

Yeah, same here.
I worked on an excel sheet.
I had 12 correct answers, and of the last 8 questions (40 slots) I had 19 slots that couldn't be.
Spoiler:
I guessed the atmospheric question as c, and tested one possibility of answers, hoping to get lucky.
I went through it, and I needed one more E.
That's when I changed 20C to 20E.

hthall wrote:I agree fully with the point made in question 20, but I was a little disappointed that the solution actually depends on this "outside information": if an alternate answer is allowed for question 20, there are four different ways to answer questions 14, 18, 19, and 20 consistently. Of course, only one of these spells a sentence and avoids answer "C"--nice touches, those.

Yeah, I didn't want Q 20 to depend on outside info either, but from the beginning I knew that questions 19 and 20 will only be answered by determining # of vowels, consonants, A,B,D, and Es.

This test was really just a "multiple-choice test taking ability test."
It tests your ability to take tests.

Right now in high school, I have often gone into a test knowing 80% of the material, and figuring out the rest from multiple choice possibilities.
So many times one question will give you so much info, that you can use it to answer 4 more questions.

Strilanc
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### Re:

Hix wrote:...
Spoiler:
Each question, of course, has exactly one correct answer. This is an important aspect of the puzzle, but it can be frustrating (and unproductive) if you fixate on this fact, insisting on always working specifically toward trying to determine which of the 5 answers is the correct one. Instead, I found it quite useful to make particular note of the fact that at least one of the possible answers to each question must be true, and seeing what I could decude from that simple fact.
...

Spoiler:
If you assume questions have exactly one answer, you pretty quickly find a contradiction. First you give 10 A and 16 D, since those are clearly the only possible answers. Then you assign A to 9, because it must have only one answer (=10). Then you can't assign any value to 13, because 9=A => 13=A, but 13=A => 9!=A.
Don't pay attention to this signature, it's contradictory.

skeptical scientist
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### Re: I dare you to solve this.

There's a difference between a correct answer and a true answer. Some questions may have more than one true answer, but only one of the true answers is correct.
I'm looking forward to the day when the SNES emulator on my computer works by emulating the elementary particles in an actual, physical box with Nintendo stamped on the side.

"With math, all things are possible." —Rebecca Watson

phlip
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### Re: I dare you to solve this.

Well, I'd describe the questions as having more than one "correct" answer, but only one "consistant" answer... you can change some questions to other answers that are "correct", but would make other questions wrong (like Strilanc's example - answering A for 9 will make 9 correct, but leave 13 without a correct answer)... and there's only one consistant set of answers that has every question correct. But this is more a terminology argument than an argument about the logical soundness of the solving methods...

Code: Select all

`enum ಠ_ಠ {°□°╰=1, °Д°╰, ಠ益ಠ╰};void ┻━┻︵​╰(ಠ_ಠ ⚠) {exit((int)⚠);}`
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reflectia
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### Re:

jestingrabbit wrote:You may or may not be in the sort of age bracket where you'd be able to solve the riddle. If you'd like to tell us about your age bracket, and perhaps a few other tasty tidbits about your life, have a look over at the introduction thread mach III: its three times the goodness.

viewtopic.php?t=10495

I don't think age is too much a factor here - I'm 15 and I got it in 30 minutes. (That might be long compared to some of you guys, though.) I found it to be a lot easier with pencil and paper, striking answers I knew to be wrong and working through by process of elimination. There wasn't too much of a problem with guesswork - I think I may have been unsure about my logic on one of the answers but everything worked out okay. When you do it out on the web, it's difficult to remember which answers you struck.
I accidentally divided by zero and my paper burst into flames.

jestingrabbit
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### Re: Re:

reflectia wrote:
jestingrabbit wrote:You may or may not be in the sort of age bracket where you'd be able to solve the riddle. If you'd like to tell us about your age bracket, and perhaps a few other tasty tidbits about your life, have a look over at the introduction thread mach III: its three times the goodness.

viewtopic.php?t=10495

I don't think age is too much a factor here - I'm 15 and I got it in 30 minutes. (That might be long compared to some of you guys, though.) I found it to be a lot easier with pencil and paper, striking answers I knew to be wrong and working through by process of elimination. There wasn't too much of a problem with guesswork - I think I may have been unsure about my logic on one of the answers but everything worked out okay. When you do it out on the web, it's difficult to remember which answers you struck.

The age thing I was referring to has to do with whether you remember a particular movie, which some people think is being hinted at by the letters that are the answers. Basically they spell out a sentence, and if you haven't seen that movie you won't get the reference and won't be able to solve it.
ameretrifle wrote:Magic space feudalism is therefore a viable idea.

reflectia
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### Re: I dare you to solve this.

jestingrabbit wrote:The age thing I was referring to has to do with whether you remember a particular movie, which some people think is being hinted at by the letters that are the answers. Basically they spell out a sentence, and if you haven't seen that movie you won't get the reference and won't be able to solve it.

You mean Fatal Attraction? I've seen it, but it isn't necessary to get the reference to solve it.
I accidentally divided by zero and my paper burst into flames.

jestingrabbit
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### Re: I dare you to solve this.

reflectia wrote:You mean Fatal Attraction? I've seen it, but it isn't necessary to get the reference to solve it.

that's what I mean and yes, it isn't necessary to solve the quiz.
ameretrifle wrote:Magic space feudalism is therefore a viable idea.