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 Post subject: riddle
PostPosted: Mon Feb 21, 2005 8:11 pm 
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Electric Sh33p
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gotta settle a bet, please help me answer this

A wizard's apprentice has four magical scrolls before him. One has a
spell he wants and the other three are empty. The apprentice chooses one of
the scrolls. His master then opens one of the scrolls the student didn't
choose to show it was blank. The master will allow for the student to
change his choice to one of the other scrolls if he wants. If the student
chooses one of the other scrolls what is the percent probability that the
scroll will have the spell?


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PostPosted: Mon Feb 21, 2005 10:35 pm 
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Baa'd to the Bone
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Location: Northeastern Iowa. In another town you've probably never heard of.
I heard a variation on this ("Let's Make a Deal" three doors instead of four scrolls) in one of my psych classes, and they said the right thing was to changing increases the probability. I don't remember why, though--it confused the hell out of me. But I suck at math and probabilities.

Kristen

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PostPosted: Mon Feb 21, 2005 11:33 pm 
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Electric Sh33p
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i know it's better to change, but I can't figure out by what percentage. At first i thought 37.5 and then I thought 50, but I'm just not sure.


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PostPosted: Mon Feb 21, 2005 11:42 pm 
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Black Sh33p
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Unless this is a trick question, I'd say there's a one in three chance that he'll get the one with the scroll on it, or roughly 33%. It wouldn't be one in four anymore, since we already know one of the four scrolls doesn't have the spell. That leaves us with three unknowns, one of them being the correct one.

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PostPosted: Mon Feb 21, 2005 11:46 pm 
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Black Sheep
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I'm going to go with 25% chance, because there was nothing said about the revealed scroll being removed, which means that it will still be a one in four chance for the apprentice.

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PostPosted: Tue Feb 22, 2005 11:07 am 
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Sh33p Dog
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This proof is in the back of the book "The Curious Incident of the Dog in the Night-Time".

I don't have the book but if you really need to know, you can go to most bookstores and look it up, it's in an appendix in the back.

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