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- Path: sparky!uunet!zephyr.ens.tek.com!uw-beaver!rice!owlnet.rice.edu!jwd2
- From: jwd2@owlnet.rice.edu (Joseph William DeVincentis)
- Newsgroups: sci.math
- Subject: Re: Problematic smurfs!
- Keywords: smurfs
- Message-ID: <BttMy8.4sM@rice.edu>
- Date: 31 Aug 92 00:06:08 GMT
- References: <94952@bu.edu>
- Sender: news@rice.edu (News)
- Organization: Rice University
- Lines: 57
-
- In article <94952@bu.edu>, spacefox@acs.bu.edu (Godfrey Degamo) writes:
- |> A group of 1000 smurfs wear either a red or blue hat on their head.
- |> They can only wear one color. The color hat on their head is known to all, but
- |> the wearer. The wearer of the hat has no means whatsoever for obtaining the
- |> color of his hat. It is never the case that all 1000 smurfs will wear
- |> the same color hat.
- |> One day the mayor, not apart of the 1000 smurfs, decides to call a
- |> town meeting. All 1000 smurfs are gathered into the town hall. The mayor
- |> is not wearing a hat. During the meeting, the mayor asks that all the
- |> smurfs wearing red hats to stand up.
- |> There is a bit of commotion in the crowd, but no one stands up.
- |> The mayor requests the same demand a bit more sternly.
- |> Again, commotion, but no one stands up.
- |> The mayor, irate, demands the red hatted smurfs to stand up.
- |> Then, a certain amount of smurfs rise.
- |>
- |> find:
- |> The exact amount of smurfs that rise.
- |> (Hint: This number will be greater than one.)
- |> The color of the hats of the smurfs that rise.
- |>
- |> more importantly,
- |> Explain how you deduced your answer.
-
- First, I will assume that all the smurfs know all the information above,
- and they know that all the other smurfs know all the information above,
- ad infinitum, and I also assume that all the smurfs are perfect logicians.
- (and that they know all the other smurfs are perfect logicians, etc., etc.)
-
- So, we begin by noting that there is at least one red-hatted (RH) smurf.
- All the smurfs know this, so if there had been only one RH smurf, he would
- have seen no other RH, and would know he had a RH, so he would stand up.
- Since no smurf stood up the first time, there must be at least two RH smurfs.
-
- If there were exactly two RH smurfs, then on the mayor's second
- command, each of them would have seen only one RH, and, noting that
- this smurf did not stand up the first time, would deduce that he himself
- also wore a RH.
-
- Similarly, on the third call, if any smurf stood up, he would only have
- done so if he saw exactly two other red hats (noting that those smurfs didn't
- stand up the previous time). Since some smurfs stood up, there must be
- exactly three of them, and all correctly deduced that they wore red hats.
-
- It's a good thing there were only three red hatted smurfs, and not 997!
-
-
-
- |> PS: please, no Delft University students' input! ;)
-
- Delft University? Where's that? ;-)
-
- --
- ----w-w--------------Joseph De Vincentis--jwd2@owlnet.rice.edu----------------
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