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A Problem Even The Wizard Can't Solve (I hope!)

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October 27th, 2011 at 3:56:46 PM permalink
boymimbo
Member since: Nov 12, 2009
Threads: 12
Posts: 2533
Geez, eh, and he posed as a Canadian, eh? Perhaps I am statman.
----- You want the truth! You can't handle the truth!
October 30th, 2011 at 7:48:22 AM permalink
weaselman
Member since: Jul 11, 2010
Threads: 17
Posts: 1924
Quote: boymimbo
Well each person is either lying or telling the truth. There are therefore 16 outcomes. The odds of each person telling the truth is 1/3. So, if all four people are telling the truth, the odds of that happening is 1/81. (1/3*1/3*1/3*1/3).

Back on topic.
Why are we multiplying these probabilities? Clearly, they are not independent. For example, if A,B, and C all are telling the truth, then the probability of D telling the truth is 1, not 1/3.
"When two people always agree one of them is unnecessary"
October 30th, 2011 at 8:32:28 AM permalink
Mosca
Member since: Dec 14, 2009
Threads: 74
Posts: 1628
Quote: weaselman
Back on topic.
Why are we multiplying these probabilities? Clearly, they are not independent. For example, if A,B, and C all are telling the truth, then the probability of D telling the truth is 1, not 1/3.


I still contend that as soon as statman misquoted the problem, this became the answer. All the other statements became dependent on the fact that D tells the truth 1/3 of the time, rather than the other way around.
NO KILL I
October 30th, 2011 at 11:37:47 AM permalink
MathExtremist
Member since: Aug 31, 2010
Threads: 46
Posts: 2521
Quote: Mosca
I still contend that as soon as statman misquoted the problem, this became the answer. All the other statements became dependent on the fact that D tells the truth 1/3 of the time, rather than the other way around.

I don't think statman misquoted the problem. The two givens are (1) A, B, C, D tell the truth with p(1/3) independently, and (2) A affirms that B denies that C declares that D is a liar. The question is "what is the probability that D is telling the truth if both (1) and (2) are true". Here's a writeup I found in Google Books:

"Eddington", by S. Chandrasekhar

Eddington calculated the result of 25/71 (correctly, under his assumptions, despite what statman said before). I arrived at 13/41 under a different set of assumptions, namely that A through D all said something relevant.
"In my own case, when it seemed to me after a long illness that death was close at hand, I found no little solace in playing constantly at dice." -- Girolamo Cardano, 1563
October 30th, 2011 at 2:36:45 PM permalink
Garnabby
Member since: Aug 14, 2010
Threads: 4
Posts: 197
Quote: MathExtremist
... if D has said nothing, the probability of D having spoken the truth is zero.


God, having said nothing to date, has never spoke the truth?

Interesting. Maybe silence is the only real truth/lie.
Why bet at all, if you can be sure? Anyway, what constitutes a "good bet"? - The best slots-game in town; a sucker's edge; or some gray-area blackjack-stunts? (P.S. God doesn't even have to exist to be God.)
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