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Everything posted by bonanova

  1. In games of this type every guess, on average, is wrong 50% of the time. For the 3-prisoner strategy, if we count the number of guesses made in all 8 cases, we find six of them are correct and six are incorrect. We "rescue" the other cases, (when both colors are seen, and no guidance is available,) with the "Pass" option. The same is true in the previous G-Y puzzle. The difference is that in the first puzzle every wrong guess produced a fatality, so survival also was only 50%. In the 3-prisoner case here, death uses up three wrong guesses (while survival still requires only a single correct guess.) We "packed" the incorrect guesses into the fewest possible of the eight cases and thus raised survival to 75%. But since 2x3 = 6x1 randomness still got its due. The 3-prisoner solution is trimmed-down version of the n-prisoner strategy. Have fun...
  2. As promised, a harder hat problem. Prisoners are seated in a circle so they can see all the others. This time the warden flips a fair coin for each prisoner and gives him a yellow or green hat, accordingly. Once all the hats have been placed, and have been seen by the others, prisoners are taken aside singly and given the opportunity to guess the color of his hat. And if instead he chooses not to guess, he is permitted to pass. Now comes the bad part. Unless at least one prisoner guesses, and all the prisoners who do guess are correct, all the prisoners will be executed. That's right, survival requires perfection from every prisoner who guesses his color. Prisoners decide on a strategy beforehand, and after the first hat is placed there is no further communication. Clearly, there can be a single "designated guesser" who ... just ... guesses a color. Half the time they all survive. But what kind of a puzzle would that be? Yes, incredibly, the prisoners can do much better. How? Maybe thinking about a three-prisoner case will answer that question. Once you're convinced they can do better than a coin toss, find their best strategy.
  3. bonanova

    Best words

    Have fun with this one. A bull stands in a pasture, unaware that he has just swallowed a time bomb that is due to explode in five minutes. Which word best describes the situation? Awful Abominable Dreadful Shocking Five minutes have passed, and all that's visible from the above scene is a sizable hole in the ground and scraps of bone and flesh. Which word best describes this situation? Amazing Silly Messy Noble
  4. What shall I do, Gerry? Asking nutty questions can be most annoying A gold key is not a common key. Horace tries in school to be a very good boy. People who drive too fast are likely to be arrested. Did I ever tell you, Bill, I once found a dollar? John came late to his arithmetic class. I enjoy listening to music at night.
  5. Can you write down a 9-letter word that permits you to erase it, one letter at a time, such that after each erasure a valid (English) word remains? (As implied, the letter order remains the same throughout.) Clue: Example:
  6. Here are nine words before, after, and in-between which only one word can be placed that gives ten different meanings to the sentence. The word is used only once, in only one space at a time, to obtain the ten different meanings. What's the only word that works? _____ TOM _____ HELPED _____ MARY'S _____ DAUGHTER _____ CLEAN _____ MARY'S _____ PARROT'S _____ CAGE _____ YESTERDAY _____.
  7. Yay! And does it seem strange to anyone else that something 7% the size of the circle on average covers the center 25% of the time? (My red herring was soooo totally ignored. Good job for doing that.)
  8. Kudos to all. @flamebirde set up an attack framework, @Izzy got the puzzle to its knees, @Molly Mae knocked it out for the count and finally @plasmid, who just might have a penchant for shooting rabbits with elephant guns, put a bullet through its head. To sort out the credits I'll don my judge's robes and presume to declare a verdict: @flamebirde told us it's all about angles. (Honorable mention. +1) @Molly Mae made four (equal) quadrants, which I have in my solution, and gave the right answer but didn't prove it, then switched to the @flamebirde - @Izzy model and backed up the answer with a word proof. (FTW) What must have been only moments later, @Izzy put numbers on things but, probably exhausted from her class, got the wrong number. (Honorable mention. +1) @plasmid dazzled us with calculus symbols and cold-blooded math to lay the newly-deceased puzzle to rest. (also Honorable mention, +1 but it doesn't seem enough. Timing is everything.) As I followed your discussion, (credit to you all) it occurred to me that this might be the simplest non-math approach:
  9. Right so far, and the one not yet calculated is interesting.
  10. This approach saves |n/2| prisoners, and that is optimal.
  11. That's it. Binary representation of B/A indicates moves to make B smaller than A was. Rinse and repeat. Nice.
  12. OK so I see MM quoting an Izzy post, but I don't see that original post. Can I invite Izzy to repost it, with complete description? Because ... it's the solution. Nice solve.
  13. Two ants named Al and Bert sit at diagonal corners of a checkerboard and decide to change places. Al, at the lower left, walks randomly upward or to the right, and Bert, at the upper right, walks randomly downward or to the left. They follow the boundaries of the checkerboard squares. That is, except when following the extreme boundary of the checkerboard, their left and right feet always touch squares of opposite color. What is the probability of their meeting (1) if they walk at the same speed, or (2) if Al walks 3 times as fast as Bert?
  14. Red herring (which I think you've figured out)
  15. Not sure if ... answer is in the title Sell to? Yes. I fixed the OP. Good catch. When I see ... right track.
  16. Place two dimes and two pennies in a line with a space between them like this: P P D D _ _ _ _ _ Using a sequence of moves, switch the two groups of coins to achieve this position: D D P P _ _ _ _ _ Moves are of two types: slide a coin to an (adjacent) empty space. jump a coin over another coin into an empty space. Type (1) move: P P D D _ _ _ _ _ Type (2) move: P D P D _ _ _ _ _ The underlines show the (only) five legal locations for the coins to occupy. What is the smallest number of moves needed?
  17. The word buried here has but one letter. Did you find a jelly roll in Gearhardt's Bakery? It's the best one I've ever seen. The rug at her stairway was made in India. He's an old friend. Amos sold his bicycle to an old friend.
  18. You've just found a neat way to place points uniformly randomly inside a unit circle: simply place points at random inside a circumscribed square -- x and y uniformly chosen on [-1, 1] -- and ignore the points near the square's corners that are outside the circle. There are other ways, but this works, and it's simple to do. And why are you excited about this? The reason is that you've often wondered about the expected size of randomly drawn triangles inside a unit circle. And now you can find out. You sequentially place a million sets of three random points in the circle, calculating (and then averaging) the areas of the million triangles they define. And you find something pretty amazing: the million triangles had an average area that is only ~ 7.4% of the circle's area. You also note that the median area was ~ 5.4%. OK, so that's a fairly long set-up for a pretty short puzzle. Read on. You tell a friend about how amazingly small random triangles constrained by a circle are, and he replies with a question of his own: "That's cool," he says, "but I wonder what fraction of those triangles cover the circle's center?" You admit that was a piece of information that you did not take note of. "Oh, that's OK," your friend replies, "I think I can tell you." What answer did your friend (correctly) come up with?
  19. Hmmm. OP does not rule out movement. But it does rule out communicating. So let's say that if the prisoners want to be at some preferred location in the room, that's permissible. But their chosen location can't be in any way influenced by hat color -- i.e., all movement must occur before the hats are placed.
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