You are drugged and wake-up in a locked room with a man. There is a table in the room and there are 11 feathers on the table. A voice booms over a loudspeaker explaining that the two of you are going to take turns removing feathers from the table. Each time you and the other chap can choose to take either one, two or three feathers. Whoever ends up taking the final feather from the table will be shot. You get to go first – how many feathers should you take from the table?
As ever, please do NOT post your answers, but do say is you have an answer and how long it took. I will post the solution on Monday.
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April 23, 2010 at 5:36 am |
Nice one. Took me a couple of minutes.
April 23, 2010 at 5:41 am |
A bit under a minute.
April 23, 2010 at 5:42 am |
Not verified my answer yet but building the logic took around half a minute.
April 25, 2010 at 5:07 pm
I’m pretty sure I arrived at my answer the same way you did.
April 23, 2010 at 5:42 am |
Wow- almost exactly a minute.. I love these- so easy in hindsight but a bear in real time. Thanks, Richard!
April 23, 2010 at 5:43 am |
Got it. Why does it have to be feathers?
April 23, 2010 at 3:55 pm
Because their captors thought that giving them grenades might cause the experiment to backfire on them.
April 23, 2010 at 5:44 am |
It took less than a minute; I ran through about 3 possibilities until I found one that worked. It assumes the other guy makes the decisions I chose for him, of course.
It will be interesting to see the real results.
April 23, 2010 at 8:01 am
The correct solution does not depend on the other guy doing what you want him to do – you can make sure he is left with just one feather if you take the right number in your first move.
April 23, 2010 at 5:49 am |
The question is indeterminate: is our goal to get shot, to get the other chap shot, or to band together to chase down the psychopath?
April 23, 2010 at 6:14 am
A very good point, Miko. I’m going with ‘chasing down the psychopath’.
April 23, 2010 at 8:11 am
I should add that, given the stated rules, there’s a solution in which no-one gets shot. Though, I am quite sure, that is not the one Richard was thinking of.
April 23, 2010 at 5:50 am |
I got it by a straightforward, linear reasoning strategy, no creative leaps.
When two feathers, A happens. When three feathers, B happens. When X feathers, doomed. When X+1 feathers, C happens. And so on.
April 23, 2010 at 6:45 am
Yes, actually it is like programming.
April 23, 2010 at 1:34 pm
Yet the strategy to follow is even more straightforward if you *do* take a “creative leap”. No need to go down the line calculating all the possibilities.
April 23, 2010 at 3:06 pm
What possibilities? If you do it the linear, non-creative way, there’s eleven lines of reasoning, each of which consists of a number of feathers and a single corresponding result/action.
April 23, 2010 at 4:23 pm
You have until Monday to figure it out.
Hints have already been given by others.
April 23, 2010 at 5:59 am |
I got it! Only a few minutes:)
April 23, 2010 at 6:04 am |
I love puzzles like this but why do they have to be morbid? Drugged, abducted, threatened with guns… it’s just unnecessary. Or have I REALLY missed something?
Partly I’m bitter because I can never do these maths ones…
April 23, 2010 at 7:02 am
Hi H, just sit back and enjoy the trip after all you are being drugged, abducted and threatened with guns…..how much better can your weekend get?
April 23, 2010 at 7:04 am
OH I know just look below and go on a sight seeing tour of Lt Cdr (R) Nazri’s Guided Missile Design site as a warm up!
April 23, 2010 at 6:15 am |
Solved in 9 minutes. I love this puzzle. It is like programming. Go to my website: GuidedMissileDesign.com
April 23, 2010 at 9:07 am
Dude, you write guided missile software in VB?
Wow. Who’d have thought that missiles have the .NET CLR installed.
April 23, 2010 at 3:59 pm
Oh not you again. Shouldn’t you be hiding in a cave somewhere unknown to every other human?
April 23, 2010 at 6:16 am |
Ooh, devious. I don’t like getting my opponent shot, but I did find a solution where I could always force him to take the last one. Eww.
April 23, 2010 at 6:19 am |
I think I got it, but it took a minute or two.
April 23, 2010 at 6:43 am
I solved it , with all possible combination. It takes about 9 minutes. All possibility.
April 23, 2010 at 6:26 am |
mmm..fun, never open the Friday puzzle before a coffee though. A couple of minutes with pen and paper to get an answer. But by taking the (last) white feather means he’s forced into cowardice?
April 23, 2010 at 6:40 am
Yes , nice puzzle
April 23, 2010 at 6:56 am |
Took me a few minutes to look for Screaming Green Conure to pluck a few feathers but can’t find him anywhere….even after a few neat bird calls……still can’t find him…..so had to chase my green cheek conure around and got bit when I tried to pluck a few feathers……darn it couldn’t come up with eleven…..even after looking in the bottom of both bird cages …. my budgie scolding me the whole time……a lass had to get pen and paper and work it out and after some trial and error {OK I know there is some sort of mathematical formula but I just don’t know it} I think I got it now…..but just in-case I ordered a bulletproof vest on line express delivery…..hope it gets here before Monday am……… :}
April 25, 2010 at 6:04 pm
Was too busy crying over how many numbers this involved, but I think I got it.
April 25, 2010 at 11:50 pm
got any feathers left on ya? hehe
April 23, 2010 at 7:04 am |
5min yee-harrrrr
April 23, 2010 at 7:04 am |
I remembered there is a mathamatical solution but couldn’t remember it so got a specific answer in about a minute then derived the general solution that would work for any number of feathers quickly from that.
In reality though I think I would refuse to take any feathers and start attacking the room for an exit.
April 23, 2010 at 7:06 am
Should have mentioned I’d be thinking of the Gom Jabar and reciting the literny against fear at the same time.
April 23, 2010 at 7:11 am |
Took me 2 minutes
April 23, 2010 at 7:17 am |
Got it in a minute, if the goal is to save myself. I’m feeling a bit sorry for the poor bastard who got shot, because I got to make the first move, though.
April 23, 2010 at 7:22 am |
about 2 minutes
April 23, 2010 at 7:26 am |
Less than a minute for all combinations – however have studied a load of maths for solving this sort of thing to find all combinations quickly. yes, just like programming.
April 23, 2010 at 7:27 am |
Now how about a n-person game with m feathers, each person taking 1..k feathers? (m,n,k being positive integers; n>1; m>n*k)
April 23, 2010 at 7:53 am |
Easy, done in less than a minute.
April 23, 2010 at 7:55 am |
Haha! I used to beat my friends at this game at school! The only guy that figured out the trick is doing masters in Physics!
April 23, 2010 at 7:58 am |
Got it almost immediately, then spent 5-10 minutes checking it. I may have come across this at school, hence getting it so quickly, but I still had to work out if I was right, as school was too many years ago to be confident about the answer.
The solution still works for 111 feathers! – You just need to be careful with your own subsequent choices.
April 23, 2010 at 8:07 am |
I think so. About 2 minutes.
April 23, 2010 at 8:09 am |
Took me a cup of tea, leave it for a few minutes, and than it took me about a minute.
Nice one, but i do not understand why people have to get killed afterwards with these puzzles, because I would probably make an agreement with the other guy just not to play the game – in order to both stay alive.
April 23, 2010 at 9:14 am
You’ll both probably get shot.
April 23, 2010 at 8:11 am |
Ik took me a little less under a minute.
April 23, 2010 at 9:07 am
Now I’m wondering if my sister reads Richard’s blog as well, or whether you just happen to share a name…
April 23, 2010 at 8:15 am |
Ha! Ha! just reread it and found away to save my and the other person from getting shot.
April 23, 2010 at 8:24 am |
Took me a minute to come up with my answer and another couple of minutes to make sure. Don’t usually come up this quickly with an answer to this type of puzzle so hoping I haven’t fallen into a trap.
April 23, 2010 at 8:25 am
Anon is me – forgot to add my name – duh!
April 23, 2010 at 8:26 am |
Finally got one
took me a while because this time double checked my reasoning.
April 23, 2010 at 8:27 am |
I sort of intuitively felt what the correct answer must be fairly quickly and then it took me another few minutes to set it out and show how it worked.
The harder these questions are the more satisfaction I get from working them out and seeing why they work.
My only problem: I’ve still not worked out how I could be in a room with ‘another man’ when I’m not a man… pedantic? moi?
April 23, 2010 at 10:53 am
Kate it seems while we were drugged we also must have had a sex change…..now that’s some trick on Mr.Wiseman or is it just an illusion? So if we are now men should we just go ahead and let them shot us and put us out of our misery?
April 23, 2010 at 10:57 am
dumb dumb should have been ” now that’s some trick ‘by’ Mr. Wiseman….” sorry for the typo!
April 23, 2010 at 8:29 am |
A couple of minutes. Helped by the fact we used to play this sort of thing at school with a whole class so used that to visualise it instead of the feathers thing.
April 23, 2010 at 8:30 am |
Dammit. I saw this one on The Real Hustle (as a bar bet, with matches instead of feathers), but i forgot the solution. Hmph. Will have to think about this when I get home from work.
April 23, 2010 at 8:31 am |
Piece of cake. . . At first i though it to difficult, but solved it within 2 min. . Hey Richard keep doing the good work
April 23, 2010 at 8:35 am |
about 10 seconds … p.s. assuming everyone uses the best strategy, the outcome is complete determined by the number of feathers at the beginning (and whose turns it is).
April 23, 2010 at 8:37 am |
Took me under a minute by gut instint; two more minutes to manually verify. But I didn’t know any maths to have figured it out!
April 23, 2010 at 8:43 am |
Got it. Started by complicating it to much and got stuck. Put the paper aside and cleared my head and started over, then it took less then a minute.
April 23, 2010 at 8:44 am |
To win is easy…..
Any solution can save both ppl?
April 23, 2010 at 8:47 am |
If you just work backwards it’s fairly easy to determine at each stage the number of feathers that you want your opponent to have left. Nice puzzle though, took a couple of minutes
Hard core puzzlers may want to try my latest puzzle 16 on my blog
April 23, 2010 at 8:47 am |
Easy. I’d took X feathers… (Oops… that can be interpreted as ten in Roman… but everyone knows that to take 10 is against the rules).
Well, the pretended answer is not difficult to get but, MY answer would have probably depend on whom was on the other side. If he was someone I like, like my father for instance, I may take the “wrong” number of feathers. Maybe Richard should have write the implicit assumption (“what feathers to take, to NOT be shot in the end?”) in an explicit form.
PS.: @Richard, I’ve started to read Quirkology and my respect for these kind of innocent queries grew up. Are you really intent to know how much time we took to achieve a solution? Or are just measuring how fast we quit from complying with that idea? I could have speculated that this has to do with the study of self-deception (measuring the averages of the answers against a controlled group), but is easy to formulate objections against it: the comments are too fuzzy since they don’t have any specific format; the group of people who comment, self-elected themselves to comment; and due to interactions, is easy to outliers to contaminate the sample (Did you note I presented 3 objections? I actually went to look for at least three arguments to present… why the need? I hope you did write about that in the chapters still to be read). Anyway, the purpose of this “postscript” is only to know if it pays off to write the initial comment in a more concise/noise free form like:
(1;Yes; ~1 minute; Had to stand up a little a think with a foot in the air; one hour)
[As you can guess, answers to (Trial number?; Do you have an answer?; How much time?; How did you processed the puzzle?; How much time did you wasted in this comment no one is going to read?)... trial number enumerates the comments in this form since in my experience, is easy to say we got the solution, and afterward, discover it is wrong]
April 23, 2010 at 8:48 am |
Took me a few minutes, good lil’ puzzle.
April 23, 2010 at 9:17 am |
About a minute. It felt a bit like playing chess
April 23, 2010 at 9:21 am |
I found an answer in a few minutes without paper, just thinking backwards from the last feather. BUT:
I’ll try to convince the other guy to split and eat the last feather so no one takes the whole feather and therefore no one could be killed for it. BUT:
I wouldn’t take any feather and as I have to start we will never come to the end. Hopefully there will finally be rescue for us. If the psychopath sends his guards in the meantime, we’ll use his own feathers to tickle them to death. BUT:
It’s a psychological test whether you use your starting privilege to get the other man (assistant of the psychologist) killed. I will shout “This is another test of the cruel Mr. Wiseman! I’m NOT at his disposal!!!” – and will pass the test.
April 23, 2010 at 9:33 am |
About 30 seconds — but I saw a very similar puzzle from when I was growing up, so I already knew the general principle involved.
(No need to enumerate all the possibilities, the solution works just as well for 99 feathers as for 11.)
April 23, 2010 at 10:03 am |
Wait… I am now recalling waking up in a dungeon with some other person. What did you do to us, Richard?
April 23, 2010 at 10:35 am |
A minute or so. If you get to make the first move, you can always win.
April 23, 2010 at 11:07 am |
As a quiz, I worked out various combinations and solved in couple of minutes. But I am very sure, if this happens in real life situation, I would not be in as clear frame of mind as I am now at my desk. And yes, I needed pan and paper. I do not think the guy whose voice booms over speaker in this quiz would be considerate enough to provide me them. And even if all the odds are in my favour, the thought of my roommate getting shot before my eyes by some psychopath is just depressing.
April 23, 2010 at 11:39 am |
We just played the game. It only took a few minutes to figure out how to win. The hardest part was our chicken was somewhat reluctant to part with feathers.
April 23, 2010 at 11:50 am |
There are two possible solutions depending on how one interprets the given scenario.
Following the first solution gets the other person shot (2 mins to work this out).
The alternative gets neither of us shot (3 mins to work this out).
Incidently I am a woman, so saying “You are drugged and wake-up in a locked room with another man…” impiles that I make a habit of it…
April 23, 2010 at 12:14 pm |
Couple of minutes thinking followed by practical verification with daughter.
April 23, 2010 at 12:27 pm |
A variant of nim.
Richard, why are you excluding your female readers?
April 23, 2010 at 12:27 pm |
Got it. 30 seconds. It’s nice.
It was easy to me because
i’m acquainted with the harder
one, with three rows with
3, 5 and 7 matches. “Nim”,
i think it’s called…
Yeah, it’s nim.
I always win. =D
April 23, 2010 at 12:53 pm |
This was quite trivial. Work backwards and the answer falls out in seconds.
April 23, 2010 at 1:24 pm |
http://www.archimedes-lab.org/game_nim/play_nim_game.html
A nim with 1, 3, 5, 7 matches.
l
lll
lllll
lllllll
I always win at that one too. ^_^
April 23, 2010 at 4:38 pm
Thanks Wisp, for introducing me to Nim math…
April 23, 2010 at 1:25 pm |
Ah, the game of Nim, everyone’s first introduction into the minimax algorithm.
April 23, 2010 at 1:26 pm |
I got an answer in about a minute and a half, but I’m not sure my logic holds up. We’ll see.
April 23, 2010 at 1:30 pm |
Took a minute or two of working through possible outcomes to figure out the principles behind the answer. Once I figured out the number of feathers I didn’t want to leave the other guy, it became pretty simple.
April 23, 2010 at 1:40 pm |
I made going backwards… no need to simulate eache combination. About 5 minutes. Great puzzle!
April 23, 2010 at 2:07 pm |
Got it, about 4 mins, used paint to illustrate it.
It is pretty cool puzzle, used to play ‘Pearls Before Swine 2′-flash game on the internet loads. It revolves around the same principle as this one but it is a bit more advanced. I used to be really good at it, but I gave it a try now and couldn’t remember how to play it and the guy in the game just laughs at me =(. Here’s one link if anyone wants to give it a try: http://www.freetheflash.com/games/pearls-before-swine-2.php
April 25, 2010 at 7:08 am
Pearls before swine is a game that I haven’t played in a while… I did find my old Excel file that guides me through the correct move in any situation. I learned the math behind it on a website… don’t have the link anymore… but it’s based on nim.
I solved the Friday puzzle quickly this time, since I knew this game. It’s a cool puzzle with lots of room for expansion.
April 23, 2010 at 2:18 pm |
10 seconds, easy one.
April 23, 2010 at 2:39 pm |
Thought about it for a minute, definitely got the right answer. Nice easy one this week.
April 23, 2010 at 4:10 pm |
Took about 5 minutes to draw out the whole thing. Very fun!
April 23, 2010 at 4:13 pm |
Whenever I get it quickly I respond and feel good about myself for an average of 5.24 seconds. Usually I’m in the seeming minority that hasn’t found the solution and feels very inadequate given the number of “so easy, took me 1 minute” responses. Then it occurred to me that we’re actually the majority. So….
I haven’t a freaking clue. I tried, I failed. I have no idea what the answer is and it actually isn’t that obvious. I can however rest assured that I’m highly trained with weapons and would have shot dead with military precision those pondering minds before they could announce their “easy, 1 minute” responses.
April 23, 2010 at 4:54 pm
Don’t worry, people always post their amazing times up. It gets ridiculous sometimes.. 1 minute? I didn’t even read it in 1 minute! lol.
April 24, 2010 at 4:02 am
Lux I could just hug you! But I would not tell you why….hehe….could be I feel your pain or understand just what you mean…..have been there my self on many occasions…..could be I like your honesty or the way you handle weapons with military precision……you could be just so darn hug-able ………or I feel sorry for you….lol…..but wait I’ll stop lol so you can concentrate on your aim once they say “easy I got it in 30 seconds”……… :}
April 23, 2010 at 4:32 pm |
I almost think this is a trick, because we have no way of knowing what the other party will choose.
Assuming he picks what I want him to pick based on my thought experiment, then it only took me a minute or two. But, then again, I think there is something to this that we are all missing.
April 23, 2010 at 4:55 pm
As far as I can tell, once you pick the right first choice, you can modify your next choices depending on what the other person in the room picks, so you always win.
April 23, 2010 at 4:32 pm |
Took me a couple of minutes doing it on my fingers ;D
April 23, 2010 at 4:33 pm |
So who can figure out what type of bird the pictured feather is likely from?
I’ll give you a hint, look at the ratio of after-feather and imagine what function it would serve…
April 23, 2010 at 4:58 pm |
I assume that my goal here is NOT to be shot?
April 23, 2010 at 5:04 pm
Ok. So I had a solution immediately, but wasn’t sure of it. Took a couple of minutes and 11 coins to verify.
April 23, 2010 at 5:03 pm |
Took me about two minutes, but I had to break out pen and paper (hangs head in shame).
April 23, 2010 at 5:36 pm |
Solved it in a few minutes when I sat down and gave it a go.
April 24, 2010 at 4:10 am
yewwwww….. really?
April 23, 2010 at 8:14 pm |
It took me about 5 minute …
Hint: Solve it backward!
April 23, 2010 at 8:20 pm |
OK, it is not so hard to find out how to solve this, but when reading this, the first thing I wondered was: How did you get drugged down, is the one you are playing against smart enough to turn the game around, making it impossible to win even if you can figure out the solution for this quiz, and why the heck do that man/woman that talks to you want to kill you, and why the heck should that person give you a game like that before he/she kills any of you, instead of just killing you fast, something that is far more plausible for someone with big enough problems in the head to actualy kill somebody.
April 24, 2010 at 4:17 am
so have ya ever seen the “saw” movies? hehe
April 23, 2010 at 8:43 pm |
hmmm….
Am I a bird? Can I just plug my own feathers?
This took me a couple of minutes to solve.
April 24, 2010 at 4:22 am
Now why didn’t I ask myself that every question? Hmmm……wait I know….IT WOULD HURT! and wouldn’t want to lose my flight feathers they could come in handy….. lol…
April 24, 2010 at 4:32 am
well got so flustered misspelled word…..darn why don’t we have “edit” control on replies???? should have been “very” not “every”……..fluff feathers and shake head….
April 23, 2010 at 8:47 pm |
I meta-puzzled to figure it out. Fun game though.
April 23, 2010 at 9:00 pm |
Oh man, I just have no idea.
April 23, 2010 at 9:21 pm |
About 3 minutes. Worked backwards in my head. Really more a matter of number crunching as opposed to the usual lateral thinking required. Have seen similar puzzles before. Love the blog.
April 23, 2010 at 10:12 pm |
Under 3 minutes. Quite good for me!
April 23, 2010 at 10:26 pm |
“You are drugged and wake-up in a locked room with a man.”
Do I want to read on!? :O
April 23, 2010 at 10:53 pm |
It’s nice.
It took me about 10 minutes.
April 23, 2010 at 11:37 pm |
I found the solution in 5 seconds. This quiz is part of a class of games called “progressively finite game”. I studied a lot about those games and I discover an interesting thing: suppose both players do their best move, the final result of the game can be “predicted” *before* the game start.
In other words, the one who start the game can easily force the victory even the other player do each time the best choose.
If someone is interested in the logic behind those games, I will explain better after richard will give the solution.
April 24, 2010 at 4:46 am
HEY LUX YOOOHOOO LUX…….NEED YOUR HELP DOWN HERE…. wow 5 seconds …..fast…… HEY LUX GOT YA A CUSTOMER…..LOL……
April 24, 2010 at 3:19 pm
Wanna play against me?
April 24, 2010 at 3:36 pm
oh no you are too fast for me…lol…
April 24, 2010 at 12:12 am |
[...] It’s the Friday Puzzle! You are drugged and wake-up in a locked room with a man. There is a table in the room and there are 11 feathers on the [...] [...]
April 24, 2010 at 12:29 am |
Talked it through with my wife, working backwards.
April 24, 2010 at 3:02 am |
Without really working it out, my gut answer is 3, so as to create an uneven number (although I know taking 1 does it as well, that creates too many opportunities to have the other person create the uneven number which would stick you with taking the last feather).
But I could be wrong.
April 24, 2010 at 3:52 am |
Couple of minutes.
April 24, 2010 at 11:51 am |
Took me about 15 seconds. But I’m a mathematician and know this one from before.
April 24, 2010 at 1:16 pm |
I’ve done this before, instead of feather, it’s books. There is some sort of pattern…
April 24, 2010 at 3:13 pm |
Think i’ve got it after about 5mins and getting out a pencil and paper. Not so confident with this one though, never mind it got my brain buzzing!
April 24, 2010 at 5:39 pm |
Within a few minutes, this was pretty easy. (:
April 24, 2010 at 6:51 pm |
Took me a while to come to a solve..
April 24, 2010 at 11:15 pm |
I have heard of a similar game to this consisting of 2 players taking in turns to remove coins. Until your opponent works out what is going on, you can make yourself appear very clever!
April 25, 2010 at 1:21 am |
Pretty easy. Rather than going to the trouble of working backwards, the easiest way is surely just to try find a counter-example for starting on 1,2,3. You’ll find counter-examples for two of those starting numbers in <10 seconds; hence it must be the other one.
April 25, 2010 at 4:31 am |
Took me less than a minute… working it out was fun… I pity the foo who gets stuck with the second turn!
April 25, 2010 at 5:25 am |
haven`t worked it out but I`m going for 2
April 25, 2010 at 1:39 pm |
Took about a minute. Good one!
April 25, 2010 at 3:22 pm |
2 seconds ;P I did this yesterday
April 25, 2010 at 6:06 pm |
Someone’s been watching Saw.
I took forever on this, AW YEAH!