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The famously difficult green-eyed logic puzzle - Alex Gendler



View full lesson: http://ed.ted.com/lessons/the-famously-difficult-green-eyed-logic-puzzle-alex-gendler

One hundred green-eyed logicians have been imprisoned on an island by a mad dictator. Their only hope for freedom lies in the answer to one famously difficult logic puzzle. Can you solve it? Alex Gendler walks us through this green-eyed riddle.

Lesson by Alex Gendler, animation by Artrake Studio.


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Автор Juan Barrera ( назад)
Yas i actually solved this puzzle

Автор FoJq Yx ( назад)
how the fuck this is a hard riddles? The probability way make this riddle a piece of cake :/ (Dissapointed)

Автор Cinloksa Nobelnioss ( назад)
This riddle could be answered a number of ways without breaking the rules, and in a few that are far more reasonable than the answer given in the video.
I'd go with the phrase, "you can all ask the guards to leave at night." Assuming the prisoners trust my words as true-- which given the video's explanation they definitely do-- they'd take it as a go-ahead, since there's no other explanation as to why I would remind them of this fact.
Frankly speaking, it makes no sense for the dictator to allow the word green in the statement I make. And any statement made which even refers to eye color wouldn't just be blown off as if it makes no difference. He'll know something is up whether you say at least 1 or at least 99.

Автор sam parrott ( назад)
id say the prisoner next to you has green eyes cause if they all do they just think well the one net to me does and im next to them. because they allready know the person next to them does so they would assume that the person next to them would assume that for themselves

Автор tailsbuddy NG ( назад)
Can't you just say "at least 99 of you have green eyes" and save a buttload of time?
I mean, they already know that information, and can use the above method without spending 99 days on the island.

Автор 房智岩 ( назад)
Why don't you just say there are at least 99 people with green eyes? So all of them can leave the second night instead of waiting for three months! If that's too bovious, say at least 50, a smaller number that won't get the dictator suspicious.

Автор The Animation's Creator ( назад)
Wait if they weren't taught language
Then what language would they think in?

Автор The Animation's Creator ( назад)
What if the kids were idiots and couldn't think of the solution
Exactly

Автор The Animation's Creator ( назад)
1:02 I would

Автор Be Studious ( назад)
they can even communicate, through actions about each other's green eyes, even though their mouths were tied

Автор That One Friend That Stans All The Members ( назад)
I would've said...
"The dictator's a dumbass, leave and be free bitches"

Автор adithya sadhu ( назад)
if one guy did not understand the logic then no one escapes

Автор De1 1Rp ( назад)
the man saying the statement should say "y'all bitches" the he proceeds to give the islander's big fuck you

Автор Steven Seaman ( назад)
pour out the water, the water is reflective so the`ll know if they have green eyes

Автор The Random Purifier ( назад)
They still dont know what green is?

Автор Joni King ( назад)
Buttt how would they all see each other ? They might not see every other prisoner

Автор Christos Tsirkas ( назад)
For all the people that cannot understand why you have to spell out something they already know:
The information here is not that "there is at least one person with green eyes". It is that they can start counting from this day forward. Since they grew up together, they do not have confidence that some particular day is the first day of the inductive logic. The only thing we have to do, is tell everybody something that will give them common knowledge that this is the first day for their reasoning.
I guess you could even say "pretend that this is your first day in the island". That would also do the trick, but it could be considered new information, so you are better off with the video solution.

Автор Candy Magic ( назад)
" If I were you I would ask the guard to leave tonight."

Автор Account thingy ( назад)
wouldn't the same logic work with the phrase "the person beside you has green eyes." since they can see each other they know this to be already true, so not to anger the dictator. however since it doesn't say which person beside on which side then they would conclude that they all have green eyes and leave that day.

Автор Bo Imphean ( назад)
Can't you see your reflection in other people's eyes?

Автор Dire Wolf ( назад)
I woulda said RISK IT!

Автор Dire Wolf ( назад)
I woulda said all potato lookin ppl have green eyes

Автор Slinja ( назад)
An easier answer would have been "Stand all in a circle and if you do not stand next to someone with green eyes walk out of it." All would keep standing in a circle and realize that therefore everyone has green eyes.

Автор Max Games ( назад)
What a dick, stealing all the recessive trained people with the very rare eye mutation.

Автор jamo90 ( назад)
i would say "water is a valuable necessity to survive" because they would know that buy now because they drink it frequently but if they are smart they would pour the water onto a table or even the ground and see there reflection and if that isn't obvious enough i would say "look into each others eyes" because you can see your own reflection if you are good enough and if they couldn't work it out from that then bad luck

Автор Monsieur Retardo Eduardo ( назад)
I would have just said "Just ask the guard to let you leave." Pretty straightforward.

Автор Corazon Cavada ( назад)
i thoght just look at yourself thought just se each other in teh eye i mean you can see a reflection in an eye

Автор taylor mortensen ( назад)
Only problem is that most people aren't that smart

Автор Magikrazy ( назад)
"You're on a remote island with 99 green-eyed people #yolo"

Автор Aura120585 Lights ( назад)
Wait, can't you just say "All of you have green eyes."?
That's one statement,and they didn't know that before,since they couldn't see their own eyes in any reflective surfaces,that's new information.
I don't get the answer either.

Автор Leleily YT ( назад)
Rlly enjoyed this simple puzzle!

Автор PC B ( назад)
They should have increased the population lol

Автор ShadowPlays 2 ( назад)
Can't they see in the ocean? There on a island... they can see there reflection in the ocean... I would say...

Look into the awesome ocean... u will SEE IT...

Автор Lj Spike ( назад)
Just an observation when viewing this again... So you tell them "At least one of you has green eyes" - I think they'd all leave on the 2nd day, regardless of how many prisoners. A key bit in the beginning of the riddle, "they all see each other in the morning headcount". That means in the morning they see everyone except themselves, therefore everyone is watching the 99 other people, and if they see anyone leave there will be a mass exodus, on the second day they realise nobody left in the night so each of them must be green eyed.

Автор Randy Meeks ( назад)
Tell them "Hug 10 people with green eyes"

Автор Macy Banmiller ( назад)
It won't load😭

Автор Hesusio ( назад)
I would've said "Everyone except you definitely has green eyes".

Автор LeoDaVee ( назад)
I would pour the water and tell them to look at it

Автор XxBeastWolfxX 12 ( назад)
I would say that 1 of the 101 of us including me does not have green eyes.

Автор Toasted Cupcake ( назад)
What if you just said "the person next to you has green eyes." then everyone would have green eyes with that logic.

Автор MoltenCheeto ( назад)
They could have looked at other prisoners pupils to see their eye color

Автор dog_lover_ #8 ( назад)
I get it with 1,2,&3 ppl bit with 100 it's confusing, plz can someone explain it 2 me PLZ!!!!!

Автор Eric Deror ( назад)
i dont get it ....

Автор Abdullah Ansar ( назад)
i have a simpler way that she said that atleast one of you have green eyes and if a person sees that every other person have same eye color then he knows that he must be the one with green eyes or everybody other have green eyes. The first one may think he have 50 chance bjt everybody can escape on the second day

Автор Captain Obvious ( назад)
Hi, Captain here. This is a run of the mill mathematical induction problem (there is a flaw into it though)
Assuming n=1, as soon as the person hears the statement, he or she immediately leave the island on the first day.
Assuming n=2, as soon as they hear the statement they look into each other's eyes. Let's say you are person A. You see person B in front of you has green eyes but you cannot see yours. So, there are two possible outcomes. You either don't have green eyes or you do. Outcome a) if you don't have green eyes then that person would have immediately left on the first night leaving you behind. However, that person stayed on the first night. So, you are certain you have green eyes and you both leave on the second night. You can expand for n=3, n=4,....n=k and so on and so on.

'Flies away'

Автор Lamir Tweed ( назад)
I would say everyone has green eyes because they know this due to the morning role call

Автор Alexander Shannon ( назад)
Same sentence, in different problem spaces, conveys different info. Example: "It is a prime number" gives immediately the solution in the set {4,5,6,8}. The same sentence causes uncertainty with the set {2,3,4,5}. Scenario is important.

Автор William Tenney ( назад)
I would have shot the dictator and then set them free but as they were on the helicopters or airplanes I would have shot them down.

If they were on a boat, I would have hidden a bomb in the boat before hand.

Автор malof ( назад)
the trick in soling this in another way is the rules if they work as like normal state rules you know that if they make a rule that rule must be 100% failsafe in any way. like you are not to walk on a sunday. there is alot of holes in this rules like remove the name sunday and you are free. just run runiining is not walking. same With sneeking is not walking. by this kolage you wil see that i am not telling you anything but showing you is different.

now onwards they say you can talk not just 1 statement Yu can talk to them therby you can ask them Things. it does not say that you cant ask them. they might not be alowed to talk to you but they can comunicate. so then i start. my barage of reasoning.

1. i tell them al to do a head motion that means yes. this way i know if my way of shaking my dead and theirs means the same or not. so noy i do the same With no. now they can say yes/no to me.

2 i dont know how mutch they know of genes but now i tell them to look at their famely or someone elses famely. notice the simularetyes. i continiue to make them understand that you get stuf from Your parents like the color of Your hair. yes you can see the color of Your hair by letting it grow. depending on their nationalety i use diferent Things that is comon among the nationalety. like if they are asian i can see if they all got the same hair color. this way they might reason that the hair color i got from my parents so my eye color i got from them to. With this reasoning cuz my parents got green so must i,

then to the parents. if my child got green eyes so do i cuz he got the eye color from us.

3 one riscy way of comunicating is to ask them if they know that water is reflective.

4. leave as a pair.
all i need to say is leave as a pair or With Your friend someone you trust.

With this you can og to Your green eyed friend and say lets leave. comon knolage. if someone ask you to leave With them and you trust them you can reason that you have green eyes cuz if not nobody wil ask you. cuz if you did not have green eyes it is the same as killing you. now you know that you have green eyes so you can leave safe. now onto you the one that asked. if you trust the one you ask and you dod not have green eyes they wil say no cuz they do not want to kill you. if you have reen eyes they wil say yes cuz there is no problem goin With a green eyed friend.

Автор srighakolapu pallam raju ( назад)
very hard riddles great

Автор shruthi s jarali ( назад)
but while drinking water they could hv seen their eye's color right (due to reflection of light in water )☺

Автор Thomas Martinez ( назад)
Or just say yes

Автор nico pwmt ( назад)
too rebuscated

Автор Sugar Kitten ( назад)
I would just risk it. p.s dumb dictator

Автор rainynight02 ( назад)
I hate things like this because few if anyone would actually think that way.

Just like with the prisoner's dilemma, where you're stuck in a cell and another person is stuck in another cell, and you have to figure out how many trees you both see. or something like that.

The whole premise of the idea only works if all parties are playing by the same rules/thought process.

If I were locked up and couldn't see my eyes, and couldn't communicate with others, and I was told "at least one of you has green eyes," when I can see that everyone else has the same eye color, and we're all from the same group... it would be safe to assume that I also had the same eye color. So if all of us are the same, and at least one of us has green eyes, I could leave right that night.

Either that, or I would now know everyone else had green eyes, but I'd be too unsure as to whether or not I too had green eyes,

Or I assume that I'm the "at least one" and everyone else is not green eyed.

Автор Moni Chan ( назад)
What if they're color blind

Автор app ( назад)
I will say you all motherfuckers just leave !!

Автор Esteban Álvarez ( назад)
Last riddle, I swear...

Автор Kawaii KAT ( назад)
I ship Adria and Bill. . Don't ask me why

Автор Kawaii KAT ( назад)
Is the mad dictator hitler ????

Автор Tyler Stacherski ( назад)
Only 2% of the world has green eyes..... and you're looking at most of them in captivity.

Автор Zackery Knotts ( назад)
She could say all of them have green eyes..

Автор CricketCo16 ( назад)
Wow!

Автор jjjakey ( назад)
"Everybody around you has green eyes"

Автор Luis theboss ( назад)
"The person to your right and left has green eyes"

Автор Kamenngls ( назад)
i would say u could escape

Автор Zainab Bashar ( назад)
If they were that clever to figure everything out, they could have just drilled a hole in the fence to gotten a glass of water to figure out their damn eyecolor.

Автор Bomen Orange ( назад)
if u r born in a remote island with only 99 others which all have green eyes, what does it make u think that u have different eye color? i mean i know on earth we all have many eye cloros cuz i see them.

for example. if on that island there were only birds withthe 100 people. they will assume that there is only 1 kind of animal in the world they live in because they haven't seen dogs or cats.

Автор 春ッチ ( назад)
This is too easy >u<

Автор Debbie Jackson ( назад)
Eyes are a reflective surface. You can see your own face in someone else's eyes. It might be difficult, but at least once, someone should have looked deeply enough to get a glimpse of their own eye colour..?

Автор Fire Lord ( назад)
wow just wow

Автор Sarah Jasim ( назад)
tbh i'd risk it and try to leave

Автор Emronaldo ( назад)
I would have said, if you see someone with green eyes, wink twice. Everyone would have left inmediately.

Автор FurryEskimo ( назад)
Wouldn't the person just learn after the first night that the other person doesn't know their eyes are green?

Автор Elite Cena ( назад)
The green eye people are not logicians. Just pour the water out of the bottle. Then slurp it up so no one notices.

Автор TheDuck369 ( назад)
Why not say "everyone around you have green eyes"

Автор Cyclone 2001 ( назад)
I love these Ted Ed brain teaser I use them in avid and they usually scare off the tutors

Автор Jeremy Tewari ( назад)
Say "You are currently living on an island"

Автор Soltaku Was here ( назад)
"If you see someone with green eyes then run up to them and slap em'

Автор Peter Willcome ( назад)
Just look at your reflection in someone else's eyes

Автор The New Dragon ( назад)
I don't understand why they would need you to tell them something they already know???? They can already see everyone else has green eyes

Автор ConnorGameGuy ( назад)
If i were one of the green eyed people i would tell everyone they have green eyes i mean they know the language, you could be like they lived there since birth maybe they havent been taught language but how else would they ask to leave

Автор digitcats ( назад)
owh ye i have green eyes :>

Автор Holo the Wise ( назад)
This is retarded. The conditions specify that there can be no new information, if no new information is given they can't conclude anything new which means that they already had enough information in the first place, of course they didn't have enough information in the first place therefore they were given new information. This is just like on of those crappy unspecified puzzles where the win condition is basically an arbitrary ass-pull of which one doesn't neccesarily need to pull this specific ass-pull.

Автор Schrödinger's Theory ( назад)
They dumb enough that they cant find whisper ay nigga you got green eyes to one a other how thry gonns figure out your stupid scenario of these guys didnt leave=I also have green eyes

Автор Nyano awx ( назад)
This video is eyesist

Автор Kashish Sood ( назад)
Your video are really logical I really like them

Автор Rosie Evans ( назад)
What about identical twins if there were any they would see the others eyes

Автор Da TRUMOFO ( назад)
What if the prisoners have BLUE BALLS?

Автор Da TRUMOFO ( назад)
This is so lame.

Автор Caydoss Entertainment ( назад)
Look at your reflection off the water !!!

Автор jinskim523 SNS ( назад)
or you could have told them to rip one of their eyes off and see if its green and if it is then go to the gaurd and ask to leave, a lot lot gruesome but it does work

Автор Ella Luv ( назад)
If you said "At least 99 of you have green eyes." It would play out like this...

Bob sees 99 people with green eyes, and thinks, "If I don't have green eyes, then everyone will leave tomorrow, knowing that they are the 99 the statement refers to." But he sees everyone else there tomorrow, so he knows he has green eyes. And everyone else goes through the same process.

If you want to go even further in depth, since 100 - 1 = 99 and there are 100 prisoners on the island, if only one person didn't have green eyes, then 99 would, and the statement refers to 99 green eyed people. So all of the others would leave knowing it's them. But they don't leave, so bob knows he has green eyes too.

If you didn't get that explanation, you should go back to kindergarten. 🍎📚📝

Автор graemenawn ( назад)
I don't see why they don't all leave on the second night? If somone sees two sets of non-green eyes then they leave on the first night. To have two sets of green eyes means your eyes are non green. Everybody is there the next day so there is no non green eyes

Автор Suso ( назад)
And what if I said: "Only one of you doesn't have green eyes"? That should work too as noone would escape on the first night. Seeing that, everyone would escape on the second night...

Автор giorgosntoulos2 ( назад)
All of you have the same eye colour?

Автор Ke Xin Li ( назад)
Wouldn't it be easier to tell the prisoners that at least 99 people have green eyes?

Автор Blade Coder ( назад)
Huzzah I got this one immediately! It is very clever.

Автор ZombehKing97 ( назад)
Actually, if you said "At least 99 of you have green eyes" that wouldn't be anything new, as the people on the island have observed the others, and have observed that everyone else has green eyes. YOU JUST GOT LOGIC'D

Автор Matthew Roh ( назад)
They are all fucking green eyed, I'd just leave.

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