BrainDen.com - Brain Teasers

# bushindo

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## Everything posted by bushindo

1. Let's say that you and 19 other prisoners are on a death row. The warden gives you and your fellow prisoners a chance to win your freedom through a game. The game is as follows, 1) When the game starts, the warden will blindfold all 20 prisoners and arrange them in a circle. 2) The warden will put on each prisoner's head either a RED or a BLACK hat. The warden will choose the color for each prisoner by flipping a fair coin. 3) The warden will then remove the blindfolds. Every prisoner will be able to see the hats on the other 19. Each person will not know the color of his own hat. There are clocks on each side of the room, so every prisoner will be able to see a clock as well. 4) The warden then says, "Everybody should be able to see the hat color of his fellow prisoners. See those clocks? For the next 20 minutes, each person may at any time raise his hand and simultaneously shout out his hat color. At the end of the 20 minutes, if everybody correctly shouted their hat colors, then you all win your freedom. Each person can only shout out his hat color once. If 1 or more person incorrectly shouted his hat color, or if 1 or more person failed to make a guess, then you all go back on the death row." 5) Each person is not allowed to communicate to his fellow prisoners during the game by varying his tone of voice, volume, facial expressions, body language, etc. The warden reserves the right to cancel the game and put everybody back on the death row if he see these types of behavior. You and your 19 friends have 1 night to discuss a strategy. Find a strategy that makes your chances of freedom as high as possible.
2. ## Quick Draw Aces

This is what I did
3. ## Quick Draw Aces

This is a really nice puzzle.
4. Brilliant and elegant solution, CaptainEd. It also has the bonus of generalizing easily to higher dimensions. I'm in awe.
5. Here's a method for generalizing the solution to the OP to any arbitrary number of dimension
6. Minor correction to the last paragraph in the spoiler.
7. Here's a discussion of why the 15 people won't all abstain during the same turn.
8. Here's an alternative method to solve the 3-D problem in the OP. Unlike plasmid's elegant solution, this one uses conditional translation and rotation. I would like some clarification. When you say 'generalize to higher dimensional space', do you mean 1) Given a random number generator that uniformly produce numbers between 0 and 1 and a description of ANY high dimensional region (e.g., a sphere in 3D, a circle in 2D, a triangle in a plane embedded within 3D space as described in the OP, etc.), produce a method that uniformly sample coordinates within the described high dimensional region. or 2) Given a random number generator that uniformly produce numbers between 0 and 1, uniformly sample the vector (x1, ...,xn ), where x1 + x2 + ... + xn = 1 and 0< xi < 1 for i = 1, 2, ..., n?
9. I apologize for posting a solution that is the same as plasmid. I read the spoiler title which said that the method is 'kludgey', and assumed that interleaving digits on numbers with infinite digits isn't kludgey, so his solution can't be the one I was thinking of. You know what people say about decomposing the word 'assume'. I feel that way right now.
10. It is possible to uniformly generate numbers in a 3-dimensional region from a single draw of a random number generator without discretizing the 3-d volume into small, finite regions.
11. Maybe i'm missing something somewhere...
12. Apparently you and I both had bugs in our codes. My bug has no other explanation besides a sloppiness on my part. Mea culpa. This board never fails to amaze me. Good work, superprismatic.
13. Great work, k-man. The exhaustive proof is a very nice bonus. This board never fails to amaze me =)
14. Ah, I see I was sloppy in phrasing this problem. By random speakers, I mean that these people randomly answer Yes or No to any question. That should bring the puzzle back along the intended line.
15. You are an explorer traveling in the Amazon rain forest. You need to explore a particularly dangerous region of the forest, so you drop by a local village of Tourguidians to hire a native as an expedition guide. The Tourguidians are the best guides to the Amazon forest. However, they are divided into 2 sub-groups: the truth tellers and the random speakers. The truth tellers will always respond to questions with the truth, while the random speakers will randomly tell the truth or lie to any question. There are 11 Tourguidians in this particular village, 6 of whom are truth tellers, and 5 are random speakers. You don't know which person belongs to which group. You would like to find a truth teller and hire him for obvious reasons ("Umm, is this herb poisonous, Mr. Tour Guide?"). However, you are only allowed to address a total of 10 questions to the villagers. Each question may be addressed to any 1 of the 11 villagers. The restrictions are that they be Yes/No questions, and that you can not ask any question which is impossible for a truth teller to answer with a yes/no (ie. asking a truth teller what a random answerer would say to a particular question, asking paradoxes, etc. ) Devise a strategy that is guaranteed to identify a truth teller in the 11 villagers using only 10 questions. Assume that each villager know the identity/persona of his fellow villagers.
16. This is a joke, right?
17. I just tested this new strategy. It works very well. Again, a generous interpretation of 'tools in the average household' is required.
19. Anyone see a mistake? Good work, EventHorizon. Nice job going beyond the call of duty. As a prize, you get both your freedom, and the 10th rat for a pet .
20. Suppose that you are a prisoner on the death row. The warden offers you a chance for freedom through a game. The game is as follows: 1) The warden shows you an array of 30 identical beakers, each containing an identical amount of clear fluid. 2) You are told that 28 beakers contain simply water. 2 beakers (you don't know which ones) contain deathly poisons which can render death in exactly 1 minute. 3) You are offered 10 live rats and 10 empty flasks. You can take turn testing for poison using the rats. 4) At each turn, you must mix fluids from any number of the 30 beakers into one of the flasks. Once done, you can feed that mixture to a rat, wait 1 min, and see if it dies or not. Only 10 turns are allowed (you can not reuse flasks or rats). 5) Assume that the poisons are sufficiently powerful that even the most minuscule amount in a mixture is sufficient to kill a rat in exactly 1 minute. Also, the poisons are very odd in that while each of them is very deathly alone, mixing them together (in any proportion) in the same flask will render the mixture completely harmless. The goal is to identify the 2 beakers containing the poison at the end of 10 turns. If you are correct, you will be set free. If not, you'll be forced to drink 10 random beakers from the 30 =). Find a strategy that is guaranteed to correctly identify the 2 poisoned beakers.
21. For a generous interpretation of 'tools in the average household'
22. Please note that there is no requirement that p(x) be a distribution with limited support (finite domain). The examples of the uniform distribution and binomial distribution that I gave have finite domain, but the gaussian example has unbounded support (-infinity to infinity). The gaussian distribution can potentially generate any number from -infinity to infinity, though certain numbers tend to show up more often. This is just as well, because while the OP states that any number from -infinity to infinity may be generated by this mysterious density distribution, it doesn't say that all numbers are equally likely to show up. In fact, we would run into trouble if the OP requires that all numbers between -infinity and infinity be equally likely. Here is a discussion on why sampling uniformly from the infinite number line is not possible (text). In post #9, I used uniform sampling on the infinite number line to show that this methodology will not improve beyond 50%. Further research into the topic has shown that uniform sampling on the infinite line is not possible, as it may lead to some logical contradiction. Please ignore post #9. The fact that the OP is can successfully generate two numbers from the unknown distribution p(x) implies that this distribution is a well-defined probability distribution. And thus well-definedness is the only requirement we impose on p(x).
23. Some comment on this approach
24. I think the fact that we can not produce a single percentage for the performance of this strategy is telling. It may be suggesting that the performance is approaching 50% from above, similar to how .9999999.... is approaching 1 from below.
25. Is it possible for the 2 generated number to be the same?
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