Subject:
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Re: math question (or pattern... whatever...)
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Newsgroups:
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lugnet.off-topic.geek
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Date:
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Mon, 3 Mar 2003 21:34:16 GMT
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Viewed:
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576 times
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In lugnet.off-topic.geek, Adrian Drake writes:
> In lugnet.off-topic.geek, David Koudys writes:
> > k, here's something that came across my desk...
> >
> > you have a bag with 4 marbles in it.
> >
> > 2 of the marbles are red
> > 1 marble is white
> > 1 marble is blue
> >
> > you reach in and grab 2 random marbles
> >
> > you open up your hand a bit to only see the colour of one of those 2
> > marbles, and it happens to be red
> >
> > what is the chance that the other marble in your hand is also red?
> >
> > I say it's 33 percent
> > my friend says it's 20 percent
>
> <long snip>
>
> you're right, in both senses. Your logic for the first solution is valid,
> and your identification of his error in the second solution (which would
> lead to the same answer) is also correct. It is, indeed, 33%
>
> Adrian
> --
> http://www.brickfrenzy.com
Thanks for the support.
Here's his rationale, hopefully explained more fully (but I still think it's
not quite kosher...)
The time of the event--you grabbed 2 marbles
there are 6 possible combinations in your hand
r(1) r(2)
r(1) b
r(1) w
r(2) b
r(2) w
b w
those are the 6 unique possibilities in your hand at the time of the event
One of those possibilities becomes invalidated because as soon as we see one
of the marbles, we know it's red so the b w is gone...
thus leaving only 5 possibilities, and what we want is the chance that both
marbles are red, from the event--
1 in 5, = 20 percent.
I understand it, but it still isn't sitting right with me.
Dave K
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Message has 2 Replies:
 | | Re: math question (or pattern... whatever...)
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| (...) I think he's wrong because he's trying to reason about partial knowledge, and that always causes problems. Look at it as a draw without replacement instead. The order you reveal things doesn't influence outcomes, they were what they were. you (...) (22 years ago, 3-Mar-03, to lugnet.off-topic.geek)
| | | Re: math question (or pattern... whatever...)
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| (...) Hmm... I just thought of something. Are there two separate questions at work here? Question 1: Having drawn two marbles from a bag of two red, one white, and one blue marbles, what are the odds that the two you have drawn are both red? (...) (22 years ago, 3-Mar-03, to lugnet.off-topic.geek)
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Message is in Reply To:
| | Re: math question (or pattern... whatever...)
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| (...) <long snip> you're right, in both senses. Your logic for the first solution is valid, and your identification of his error in the second solution (which would lead to the same answer) is also correct. It is, indeed, 33% Adrian -- (URL) (22 years ago, 3-Mar-03, to lugnet.off-topic.geek)
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