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Post » 18 Oct 2014, 01:07

· RIDDLES & BRAINTEASERS ·Move to Shameless Chamber if necessary This is the official thread for Riddles and Brain Teasers.
Your objective is to solve the riddles cooperatively.
Once a riddle/brainteaser is successfully finished, the next person may post a new one.


  1. No Cheating. That means do not search for solutions related to the riddle. That completely destroys the point of the riddle itself.
  2. If you think you found the solution/answer, put it in a spoiler. It may be the correct one, but we never know.
  3. Non-serious solutions/answers will be ignored.
  4. If you didn't make the riddle, you must give appropriate credit to the creator.
  5. No hints can be said.
  6. If you want to post a riddle, make sure you know the answer. So you can say if it's the right answer or not.

First Riddle:

Code: Select all

A group of people with assorted eye colors live on an island. They are all perfect logicians -- if a conclusion can be logically deduced, they will do it instantly. No one knows the color of their eyes. Every night at midnight, a ferry stops at the island. Any islanders who have figured out the color of their own eyes then leave the island, and the rest stay. Everyone can see everyone else at all times and keeps a count of the number of people they see with each eye color (excluding themselves), but they cannot otherwise communicate. Everyone on the island knows all the rules in this paragraph.

On this island there are 100 blue-eyed people, 100 brown-eyed people, and the Guru (she happens to have green eyes). So any given blue-eyed person can see 100 people with brown eyes and 99 people with blue eyes (and one with green), but that does not tell him his own eye color; as far as he knows the totals could be 101 brown and 99 blue. Or 100 brown, 99 blue, and he could have red eyes.

The Guru is allowed to speak once (let's say at noon), on one day in all their endless years on the island. Standing before the islanders, she says the following:

"I can see someone who has blue eyes."

Who leaves the island, and on what night?

There are no mirrors or reflecting surfaces, nothing dumb. It is not a trick question, and the answer is logical. It doesn't depend on tricky wording or anyone lying or guessing, and it doesn't involve people doing something silly like creating a sign language or doing genetics. The Guru is not making eye contact with anyone in particular; she's simply saying "I count at least one blue-eyed person on this island who isn't me."

And lastly, the answer is not "no one leaves."

Have fun!
I know the god damn answer because of the solution page. I'm not some super master mind guy.

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Post » 18 Oct 2014, 02:46

 The ferry leaves. ?

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Post » 18 Oct 2014, 03:03

WaryLouka wrote:From XKCD
Oh hey sweet!
How'd you find this? I never came across it in the webcomic itself or What If or anything...

Also btw I will definitely give this a good read and analysis after solving the problem I'm trying to solve right now*. Thanks for sharing :D

*Which I am legitamately doing - this isn't some joke like "the problem of my lack of caring" or whatever sarcastic thing you probably expect from me by now sorry to disappoint :P
If you must know it involves a bottle wrapper in the shape of an isosceles triangle that was folded up in my pocket, and deriving a math formula. Because that's what I do in my free time

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Post » 19 Oct 2014, 22:13

can I post a riddle from an MIT test :D?

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Post » 19 Oct 2014, 23:44

Luke wrote:can I post a riddle from an MIT test :D?
After the current one get solved, which apparently has a big lack of progress.

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Post » 19 Oct 2014, 23:53

All the blue-eyed people?

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Post » 20 Oct 2014, 01:25

Why would you post that puzzle on a forum full of teenagers to actually attempt to solve. Maybe as an example, but you can't seriously expect anyone here to go think up the solution. I had to read it twice to see why the solution even works, it would take me ages to conclude that.

Start with a different one.

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Post » 20 Oct 2014, 01:35

Sašo wrote:Maybe as an example, but you can't seriously expect anyone here to go think up the solution.
Logic puzzles are a huge interest in my family - I'm having fun trying to solve this (my dad is too - I showed him). I can see your concern, but I wouldn't completely underestimate the forum as a whole unless this is something straight out of the depths of hell or something (which it may be - I don't know the answer to estimate how difficult it is :P)

Maybe as a compromise we could have multiple puzzles going at the same time? I was going to suggest that earlier but was too focused on this to remember to ask.

Also Wary you never answered me. I haven't seen this through the main xkcd page anywhere.

EDIT: @Turret and MagicPillow: You aren't specifying what night they leave, or any explanation as for why
You should probably think through your answers

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Post » 20 Oct 2014, 07:43

 The captain of the ferry leaves with his ferry every night? 

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Post » 20 Oct 2014, 10:24

Dav999 wrote: The captain of the ferry leaves with his ferry every night? 
Wary forgot this part in xkcd's logic puzzle:
xkcd wrote:I've done my best to make the wording as precise and unambiguious as possible (after working through the explanation with many people), but if you're confused about anything, please let me know. A word of warning: The answer is not simple. This is an exercise in serious logic, not a lateral thinking riddle. There is not a quick-and-easy answer, and really understanding it takes some effort.

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Post » 20 Oct 2014, 15:57

I might be able to explain it a bit more clearly than XKCD. No promises.

Eye color spoilers
6 brown eyed (BR), 4 blue eyed (BL) - to make the example easier to explain. It works regardless of the number of blue and brown eyes. However, there has to be at least 1 with blue eyes that the Prophet can see.

Their assumption is the following (let's say on the first day):
BR: I see 4 BL people. If I have BL, there are 5 BL people.
BL: I see 3 BL people. If I have BL, there are 4 BL people.

The marked red are the "incorrect assumptions", the unmarked are correct.
The 1 man difference by each's wrong assumption is the key.

Since anyone can leave, the situation is as following:

Day 1: no one leaves.
Day 2: no one leaves.
Day 3: no one leaves.
Day 4: BL: I see 3 BL, but they have not left on third day. If there were a total of 3 BL, they would have each counted 2 and then left on the third day. That must mean I have BL.
BR: There are 4 BL.
All BL leave.

That is all.