Forums → Forum Games → Riddles
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So, there was a thread on here a while back dedicated to riddles. Thaboss hasn't seen any new thread recently, so he's decided to start his own. Feel free to post and respond to any riddles.
Here's a classic one.
Bob hates his job. One day, while Bob is working on the 34th floor of an office building, he decides he's fed up with it. He rips open a window, and jumps through it. Although Bob is on the 34th floor, and there was nothing to stop, slow down, or cushion his fall, when he landed, he was perfectly fine. How?
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Alright, thaboss heard this one, but forgot the exact wording. So he googled it and found it, but he's just gonna copy/paste. So please don't google this because you WILL find it.
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. If anyone has figured out the color of their own eyes, they [must] leave the island that midnight. 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?
To quote,
"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."
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."
Why do you keep posting these?! This isn't a riddle, it's a logic puzzle!
True, but thaboss likes logic puzzles. Those were his only two anyway.
Matt is correct.
The answer is that on the 100th day, all 100 blue-eyed people will leave. It's pretty convoluted logic and it took me a while to believe the solution, but here's a rough guide to how to get there. Note -- while the text of the puzzle is very carefully worded to be as clear and unambiguous as possible (thanks to countless discussions with confused readers), this solution is pretty thrown-together. It's correct, but the explanation/wording might not be the best. If you're really confused by something, let me know.
If you consider the case of just one blue-eyed person on the island, you can show that he obviously leaves the first night, because he knows he's the only one the Guru could be talking about. He looks around and sees no one else, and knows he should leave. So: [THEOREM 1] If there is one blue-eyed person, he leaves the first night.
If there are two blue-eyed people, they will each look at the other. They will each realize that "if I don't have blue eyes [HYPOTHESIS 1], then that guy is the only blue-eyed person. And if he's the only person, by THEOREM 1 he will leave tonight." They each wait and see, and when neither of them leave the first night, each realizes "My HYPOTHESIS 1 was incorrect. I must have blue eyes." And each leaves the second night.
So: [THEOREM 2]: If there are two blue-eyed people on the island, they will each leave the 2nd night.
If there are three blue-eyed people, each one will look at the other two and go through a process similar to the one above. Each considers the two possibilities -- "I have blue eyes" or "I don't have blue eyes." He will know that if he doesn't have blue eyes, there are only two blue-eyed people on the island -- the two he sees. So he can wait two nights, and if no one leaves, he knows he must have blue eyes -- THEOREM 2 says that if he didn't, the other guys would have left. When he sees that they didn't, he knows his eyes are blue. All three of them are doing this same process, so they all figure it out on day 3 and leave.
This induction can continue all the way up to THEOREM 99, which each person on the island in the problem will of course know immediately. Then they'll each wait 99 days, see that the rest of the group hasn't gone anywhere, and on the 100th night, they all leave.
Monty Hall problem. Always switch your door. That way you have 50/50 odds instead of 1/3.
Yeah. You're right. 2/3.
How quickly can you find out what is so unusual about this paragraph? It looks so ordinary that you would think that nothing is wrong with it at all, and, in fact, nothing is. But it is unusual. Why? If you study it and think about it, you may find out, but I am not going to assist you in any way. You must do it without coaching. No doubt, if you work at it for long, it will dawn on you. Who knows? Go to work and try your skill. Par is about half an hour.
Another good one.
Thaboss has a 12 page file of riddles collected over the ages, including many from the old riddle thread. It would be cool if everyone could collaborate all their riddles into one massive file... Any idea how?
Oh no, thaboss meant like for if others have lists of riddles and would like to combine with thaboss. He's already taken all riddles from this thread.
http://sdrv.ms/WU6wn3
Here's the file. Thaboss made it editable. Try not to look at too many of the other riddles. Unfortunately the 53 eagles one is not there, just the 53 bicycles. Sorry. Also the ones at the beginning are more silly because he started the list a long time ago.
I wana give a riddle too
One Verry rich man was bored.. He through about, what was missing in his life.. Something that he never did.. And he finnaly found it. He had never went to the moon!!!! He immediately spend billions to buy a spaceship. On the next morning he was ready to fly when the night guard stopped him, and told him that the spaceship will crash and he will DIE!!! The rich man was verry into the ocult so he through it may happen so he went home instead of going on the spaceship. Some other rich guy, bought the spaceship. That nigh the Rich man was watching news and what to see !! The spaceship he was gonna fly with, has exploded into the air. The Rich Man immediately went and thanked the guard, then he fired him. WHY???
My autocorrect does not work... It underlines all words in red.... And no, If he wanted to kill the witnesses, he would kill the guys he bought the spaceship from.
Okk here is a hint - The guard told him, he dramed it. It was a Dream of The Future !!!!! Future Vision!
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