How'd they figure that? (logic puzzle)

  • Context: Undergrad 
  • Thread starter Thread starter belliott4488
  • Start date Start date
  • Tags Tags
    Figure Puzzle
Click For Summary

Discussion Overview

The discussion revolves around a logic puzzle involving a population of citizens on an island who must deduce the color of their own eyes based on certain rules and a revelation from a stranger. The focus is on the reasoning process behind their eventual actions and the implications of the established laws governing them.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants discuss the implications of the stranger's revelation and how it leads to the eventual suicides of the green-eyed citizens after 800 days.
  • Others propose variations of the scenario, such as the case where only green-eyed people exist and how the introduction of a blue-eyed person would change the outcome.
  • A few participants challenge the order of the rules and their implications, questioning how the act of not committing suicide could reveal eye color and whether this violates the second rule.
  • Some express confusion over the logical deductions that lead to the conclusion of suicide, particularly regarding the timing and reasoning involved.
  • There is a discussion about the potential contradictions between the rules and how they affect the citizens' actions.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of the rules and the logical deductions involved. There is no consensus on the implications of the rules or the exact reasoning process that leads to the suicides.

Contextual Notes

Some participants note potential typos in the problem description and discuss the need for clarity regarding the sequence of events and the application of the rules.

Who May Find This Useful

Readers interested in logic puzzles, mathematical reasoning, and discussions on deductive reasoning may find this thread engaging.

belliott4488
Messages
661
Reaction score
1
Okay, here's my favorite brain teaser. Sorry if an equivalent version has appeared here before.

On a remote island somewhere there was an ancient country whose population consisted of 200 people with green eyes and 800 people with blue eyes. The citizens of this country were all happy and lived harmoniously, dutifully obeying the only two laws that governed them:

1: At noon every day, all 1000 citizens must gather in the town square for ten minutes of silent contemplation. If, at the end this period, any citizen knows the color of his own eyes, he must immediately commit ritual suicide.

2. Under no circumstances is any citizen ever to speak of or otherwise make any reference to eye color, his own or anyone else's.

Naturally, given the dire nature of the first law, there were no mirrors or other reflective surfaces to be found anywhere on the island, nor any other means for a citizen to discover the color of his eyes accidentally.

Life went on peacefully for years, since no ever learned or wanted to learn the color of his eyes. All that ended, however, after the fateful day when a stranger washed up on shore, the only survivor of a doomed pleasure cruise. As he staggered up on to the beach, he looked about him and declared, "whoa, cool - there are dudes with green eyes here, man!" Naturally, the citizens were shocked by this breach of protocol, and threw stones at the stranger, driving him back into the sea, where he was promptly devoured by a hungry shark.

Now, since he did not specify which people had green eyes, no one on the island had learned his own eye color, so no one committed suicide the next day. Or the next, or the next after that. This went on for a long time, but on the 800th day after the stranger's arrival, all the green-eyed people committed suicide after the town meeting at noon.

The question, of course, is how did they know their eyes were green? Secondarily, why did it take 800 days after the only revelation of new information (which wasn't really new, since everyone on the island already knew that there were people with green eyes on the island)?

This isn't too difficult if you've seen this type of problem before, but it is kind of fun to work through the necessary thought processes of these poor doomed simps.
 
Mathematics news on Phys.org
nice problem, i enjoyed that one

even more interesting:

lets say there are only people with green eyes on the island (no blue-eyed people). after the stranger arrives and delivers his edict, the island population will be fine, free of suicides...until a single blue-eyed person arrives.

(btw, you have a typo in your description of the problem, they all commit suicide 200 days after the stranger's arrival - the number of blue-eyed people is irrelevant , there need only be at least one blue eyed person).

also, to be complete, i would say that rules 1 and 2 must be evaluated in that order. technically you could argue that by committing suicide you would be indirectly telling someone their eye color! having rule 1 preempt rule 2 prevents that.
 
Last edited:
quetzalcoatl9 said:
nice problem, i enjoyed that one

even more interesting:

lets say there are only people with green eyes on the island (no blue-eyed people). after the stranger arrives and delivers his edict, the island population will be fine, free of suicides...until a single blue-eyed person arrives.
Really? I would have thought that after a number of days equal to the population, everyone would have to commit suicide, each citizen having figured out that he's not the single blue-eyed person on the island.

quetzalcoatl9 said:
(btw, you have a typo in your description of the problem, they all commit suicide 200 days after the stranger's arrival - the number of blue-eyed people is irrelevant , there need only be at least one blue eyed person).
Aughh! You're right - I messed that up! I hope I didn't confuse anyone and prevent his or her solving the puzzle ... :rolleyes:
quetzalcoatl9 said:
also, to be complete, i would say that rules 1 and 2 must be evaluated in that order. technically you could argue that by committing suicide you would be indirectly telling someone their eye color! having rule 1 preempt rule 2 prevents that.
I'm not sure I'm understanding what you mean by evaluating the rules, let alone the order in which they're done. The second rule applies at all times, and the first applies once ever day. Why would you have to think of one applying first?
Also, you are right about suicide informing others about their own eyes. In fact, if the citizens are aware that there are only two eye colors available, all the blue-eyed citizens would have to commit suicide the day after the green-eyed guys. If there were the possibility of brown-eyed people, then I guess they'd still be okay.
 
belliott4488 said:
Really? I would have thought that after a number of days equal to the population, everyone would have to commit suicide, each citizen having figured out that he's not the single blue-eyed person on the island.

yes you're right, scratch that, i don't know why i thought that..

belliott4488 said:
I'm not sure I'm understanding what you mean by evaluating the rules, let alone the order in which they're done. The second rule applies at all times, and the first applies once ever day. Why would you have to think of one applying first?
Also, you are right about suicide informing others about their own eyes. In fact, if the citizens are aware that there are only two eye colors available, all the blue-eyed citizens would have to commit suicide the day after the green-eyed guys. If there were the possibility of brown-eyed people, then I guess they'd still be okay.

the act of NOT committing suicide will reveal someone's eye color, which is prohibited by rule 2. if rule 2 took precedence over rule 1, then i would not follow rule 1. as long as rule 1 takes priority over rule 2 there is no conflict. this is similar to how robocop went nuts :)

also, we could say that they commit suicide "if and only if" they discover their eye color, thus disallowing suicide for any other reason.
 
quetzalcoatl9 said:
the act of NOT committing suicide will reveal someone's eye color, which is prohibited by rule 2.
Sorry ... still not getting it. Before the revelation by the stranger, life went on with no suicides and no problems - how would the lack of suicides have indicated anyone else's eye color, and whose eyes would those have been?

Or, are you saying that would happen only after the stranger's revelation? I guess on day N-1, the fact that all the green eyed guys didn't do themselves in would, in fact, reveal to the other greenies what their eye color was. Of course, that's how they know to commit suicide the next day ... so maybe I do get it - there's a fundamental contradiction between rule #2 and any action or inaction that allows anyone to deduce the color of his eyes, so suicide must always result from someone having violated Rule #2. Of course, we could also take a more literal reading of that rule, which would be to say that no one may overtly or explicitly discuss anyone's eye color, but merely failing to prevent someone else from a logical deduction doesn't count.
quetzalcoatl9 said:
also, we could say that they commit suicide "if and only if" they discover their eye color, thus disallowing suicide for any other reason.
Yeah, :-p that would help, too!
 
belliott4488 said:
Or, are you saying that would happen only after the stranger's revelation? I guess on day N-1, the fact that all the green eyed guys didn't do themselves in would, in fact, reveal to the other greenies what their eye color was. Of course, that's how they know to commit suicide the next day ... so maybe I do get it - there's a fundamental contradiction between rule #2 and any action or inaction that allows anyone to deduce the color of his eyes, so suicide must always result from someone having violated Rule #2. Of course, we could also take a more literal reading of that rule, which would be to say that no one may overtly or explicitly discuss anyone's eye color, but merely failing to prevent someone else from a logical deduction doesn't count.

yes precisely
 
you guys think way too advanced for me...

*quits forum*
 
omg precal said:
you guys think way too advanced for me...

*quits forum*
ha ha - that was something else I forgot in the original post: the citizens of this country all think way too much!
 
  • #10
belliott4488 said:
ha ha - that was something else I forgot in the original post: the citizens of this country all think way too much!

yes, but it is to the citizens detriment! they would be better off if they were not so rational!

or, alternatively, they would be better off not living within a theological system :smile:
 
  • #11
hey but before all of that, could you just explain the problem in a simple way first, they you can all argue about the detail. i kinda get it... that because after 200 days, the green found out that they must be green.

but if these people can count, and these people think too much. they could simply count the number of green people in the society and know their own color assuming that they know that there have to be 200 blue and 800 green and 1000 total...
 
  • #12
ArielGenesis said:
but if these people can count, and these people think too much. they could simply count the number of green people in the society and know their own color assuming that they know that there have to be 200 blue and 800 green and 1000 total...

but they don't know exactly how many people have green eyes, they only know what they see around them
 
  • #13
could someone spell it out for me?
 
  • #14
The best way to understand it is to use mathematical induction, i.e. figure it out for a small number and then show that the reasoning extends to larger numbers. So in this case, start with two blue-eyed guys and one green-eyed guy. Once you're convinced that the green-eyed guy must die on day 1, see if you can see why two green-eyed guys would have to die on the 2nd day. Carry on from there.
 
  • #15
A. Why are they trying to kill themselves? You all assume they want to know what colour their eyes are and want to commit.

B. I don't get either part of the question.
 
  • #16
Wonderballs:

This is a logic puzzle, not a study in sociology. The obviously absurd premises of the laws of this community and the citizens' willingness to abide by them are simply given. The task is to determine the logical reasoning that would allow the citizens to reach the conclusions they do, when they do. It's not about why people behave in any particular way - that's just a humorous construct.
 
  • #17
I still don't understand why after 800 days they commit, or after the foreigner gets washed away they suddenly know what colour theier eyes are even though it stated that he didint specify whose eyes were green.

Which is exactly why its a brain teaser
 
  • #18
You're right - that's the puzzle! ;-)

Did you try my hint in an earlier post? That's a good way to start.
 
  • #19
I have been looking over this in my head for quite some time, and I just can't figure it out. I'm not convinced that even if ONE of them commited suicide that the rest would follow suite... and I definitely don't understand why they ALL do it on the 200th day.
 
  • #20
spoiler:



consider the case of only 2 ppl, one having green eyes. pretend that you are one of them - if you see that the person across from you does not have green eyes (and you know that at least one person has green eyes) then there is only one possibility - you have green eyes. therefore, you do yourself in the next day. so, 1 person had green eyes and then commit suicide 1 day later. (note the number of ppl with blue eyes is irrelevant).

consider the case of only 3 ppl, with 2 ppl having green eyes. let's say that you have green eyes: you will look around and see 1 person with green eyes and 1 person with blue eyes. now there are only two possible choices here (a) you have green eyes or (b) you have blue eyes. if you have blue eyes, then there is only one person on the island with green eyes in which case we have the situation described in the paragraph above. but if you wait one day and that other green-eyed dude did not commit suicide, then you know that he must be seeing another green-eyed person out there - YOU! therefore you will kill yourself after 2 days, as will he since he is in exactly the same boat. so, 2 people with green eyes and then commit suicide 2 days later.

continue for N people and N days...


[/color]
 
  • #21
If the people either know that there are 800 blue or 200 green couldn't they have just walked around counting before the stranger comes, ie if they count only 199 green they are green or count 799 blue they are blue
 
  • #22
There are alternatives.

1. Could always pluck out your eyes. Then you wouldn't have green eyes any more and wouldn't be in violation of the rules when the time came for the ritual.

2. Could just assume that the cast away was a dirty liar who likes to cause trouble for communities by breaking the rules. (ofcourse I would conveniently assume he followed the same rules I did.

3. Change the laws.

Would the following options work?

4. Could sacrifice yourself at the first ritual the following day, sparing everyone else from discovering their eye color.

5. There could be an new villager with green eyes, such as a newborn, who hadn't heard the castaways words.

Honestly, I don't understand why would all kill themselves after 200 or 800 days. If they had never seen anyone with green eyes and heard that comment then I can understand they would know they had green eyes. But since they can see 199 or 200 other people with green eyes, then ofcourse they already know that some people have green eyes. In this case the strangers words aren't revealing anything new. Why would him telling them what they already know start mass suicides?
 
  • #23
If I see 0 people with green eyes and I discover that someone has green eyes, then I can be sure that I am the one with green eyes.

If I see 1 person with green eyes and he does not kill himself at the first ritual then I know that there must be more than one person with green eyes. If I see nobody else with green eyes then I know I have green eyes.

If I see 2 people with green eyes then at the first ritual I would assume they see either 1 or 2 people with green eyes, with myself being that possible 2nd person.

How would I know if I had green or blue eyes after the second ritual when I see 2 people with green eyes? How can I conclude that if they don't kill themselves, I must have green eyes? How does the number of rituals factor into the logic?
 
  • #24
see my post #20 above, and all will be told...
 
  • #25
I explained as much in my post, but I still don't understand how it applies when I can see 2 or more people with green eyes.

And I still don't understand why some castaway telling them what they already know would begin this fatal chain reaction. I would think day 0 would begin from the moment they passed the two laws. Seems a ridiculous thing to do, sentencing themselves to death like that.
 
  • #26
Huckleberry,
No one is suggesting that this is a true story nor that it would make much sense in the real world. It's a logic puzzle.

Here's the answer to your question re: seeing two green-eyed people:
You've figured out what your response would be if you saw only one green-eyed person. If you saw two green-eyed people, then you'd hope that they each see one green-eyed person and will do exactly as you would in that case, i.e. they'd kill themselves on day 2. If they don't, then the only explanation is that they each see two green-eyed people, not just one. You must be the second one. You learn this on day two, so on day three you do yourself in.

The same logic applies on day 3 if you see 3 green-eyed people: if you're blue-eyed, then the green-eyed guys kill themselves on day 2 as you would have in the previous paragraph; if they don't, then they must see three green-eyed people, so you must be green-eyed.
 
Last edited:
  • #27
belliott4488 said:
Huckleberry,
No one is suggesting that this is a true story nor that it would make much sense in the real world. It's a logic puzzle.

Dude, I totally thought this was for real! :eek: You scared me.

I know it's a logic puzzle. I'm saying I think the puzzle is broken. The addition of the mysterious stranger who washes ashore one day to reveal to the villagers that some of them have green eyes creates a paradox. Since the villagers have been living with these laws for a long time and they already know that some people have green eyes, by the time the guy washes ashore there would be 0 or 1 person on the island that has green eyes.

So, obviously the guy is a liar, and people shouldn't commit suicide based on what he says. As far as I can see the arrival of this stranger has no meaning to the logic puzzle. The purpose of adding him seems to be the same as a guy walking into the room when there are only 0 or 1 people with green eyes and letting everyone know that someone has green eyes. I don't how this works when there are already 2 or more people with green eyes.

How would this puzzle work with no mysterious stranger? Or can you explain to me why the stranger is necessary in this puzzle?

edit - since the guy doesn't specify how many people have green eyes I don't see how what he says makes any difference. And since the villagers don't speak of their eye color they would be perfectly safe. Now if he said 200 of you have green eyes then 200 people would commit suicide the next day. Otherwise the paradox that they are alive at all would continue.
 
Last edited:
  • #28
And what's stopping these people from looking around and saying to themselves, "Hey, I see 199 other people with green eyes. Certainly they must see either 199 or 200 people with green eyes also. So if they don't all kill themselves tomorrow then I must have green eyes also and we can all kill ourselves the day after that.

Why do they need to wait 200 days at all? I think the puzzle is broken because they know if everyone else has green eyes. They only don't know what color their own eyes are.
 
  • #29
Well, Mr. Huck, you've certainly got the problem down! Your questions are exactly the statement of the problem: how can it be (logically) that these guys kill themselves when they do, given all the considerations you've named? That's precisely what makes this an interesting problem - to me, at least.

I think the only problem you're having is that you haven't yet fully absorbed the logic that leads to the stated result. I'll give it another shot here (more spoiler):

Suppose there's one green-eyed guy. So long as no one says, "hey, there's a green-eyed guy here," he never knows his eyes are green, nor do any others know that their eyes aren't green. The significance of the arrival of the stranger is that he does, in this case, bring a new piece of information to one person, namely the green-eyed guy, who now knows that 1) there's a green-eyed guy and 2) he's it.

Now suppose there are two green-eyed guys. Yes, they both know that there's at least one green-eyed guy, just as everyone else knows that there are at least two. No one knows the exact number, though, not knowing his own eye color. Now the stranger arrives and makes his announcement. Each green-eyed guy looks at the other one (surreptitiously), hoping that that guy is the only green-eyed guy so that he'll kill himself on day one, for the reasons in the preceding paragraph. They both eagerly await this on day one, but of course, neither one kills himself, so they are both very disappointed. That's because each one knows that the only reason the other green-eyed guy didn't kill himself must be because that guy himself sees another green-eyed guy, who must be himself. That's how they both discover that they are green-eyed. Everyone else, by the way, was eagerly hoping to see both of them kill themselves on day 2, which is how they would learn that they were not green-eyed.

If there are three green-eyed guys, they each wait until day two to see if the two green guys they see will kill themselves as in the previous paragraph. When they don't, then each of them realizes that he must be a third green-eyed guy.

The same reasoning applies for N green-eyed guys on the Nth day.

It is truly weird, since you can well ask, as you did, why should they all wait till the 200th day, since they all know that there are at least 199 green-eyed people? The answer is, how would they agree on this? They couldn't say how many green-eyed people they see without immediately letting everyone else know whether or not he had green eyes. So, maybe they could just pick a safe number, say 2 less then the number they see. Well, the blue-eyed guys might think it's safe to say, "let's skip the first 198 days, since everyone agrees that we all see at least 198 green-eyed people, right?" But that immediately gives away the fact that they see 200, because each green-eyed guy sees only 199, so each of them would have said, "we can agree to skip 197 days, since I know everyone see at least 197." But if he were smart enough, he wouldn't even say that, because he'd realize that if there really were only 199 green-eyed people (i.e., if he were not green-eyed), then each of the 199 green-eyed people would be wondering if there were really only 198 green-eyed people, so that those guys would recommend skipping196 days -- except that they couldn't, because they'd be wondering if those 198 green guys were seeing 197 green guys, in which case they would suggest skipping 195 days, and so on and so forth ...

It's called recursion, and it's not easy to get your brain to wrap around it. I once forced myself to write out the thought process of each green-eyed guy on Day 199, just as I did above for the simpler cases above - it's not easy, but it is logical! :-p
 
  • #30
are you people really that slow, or just can't read?? there IS new information being introduced by the stranger, which is the following:

"there is at least one person here with green eyes"

which was NOT known before the stranger's arrival
 

Similar threads

Undergrad Video on imaginary numbers and some queries
  • · Replies 3 ·
Replies
3
Views
2K
Undergrad A new puzzle based on the "Blue-eye paradox"
  • · Replies 19 ·
Replies
19
Views
2K
Graduate How Did the Student Solve the Green Hat Puzzle?
  • · Replies 21 ·
Replies
21
Views
10K
Coding Bootcamps - Can Anyone be a Six-Figure Coder?
  • · Replies 11 ·
Replies
11
Views
4K
Undergrad Lorentz Transformation: Premises + Derivation
  • · Replies 5 ·
Replies
5
Views
2K
Medical Passed out while giving blood-recovery period?
  • · Replies 3 ·
Replies
3
Views
2K
Graduate Blue-Eye Paradox: Solution Not Unique
  • · Replies 276 ·
10
Replies
276
Views
28K
Undergrad Unidentified Luminescent Phenomena (ULP)
  • · Replies 1 ·
Replies
1
Views
4K
Undergrad Where can I find publicly accessible data from observatories in Europe?
  • · Replies 5 ·
Replies
5
Views
2K
Did the 1883 Krakatoa Explosion Leave Ships in Peril?
  • · Replies 16 ·
Replies
16
Views
7K