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#1 2013-06-11 07:33:02

mom
Member
Registered: 2012-04-25
Posts: 94

balls in an urn

An urn contains 3 yellow marbles and 5 green marbles. One marble is removed, its color noted and not replaced. A second marble is removed and its color noted.

What is the probability that both marbles are yellow
Pr(YY)=3/28
I did 3/8*2/7=3/28
What is the probability that they are both green?
Pr(GG)=5/14
I did 5/8*4/7=5/14
what is the probability that exactly one marble is yellow?
there were two chances of this
I did 3/8*5/7=15/56 and 5/8*3/7=15/56
Do I divide by two for the probability that exactly one marble is yellow?

I am not sure if I have done all of the steps for any of the questions. Can someone look over my work and give me advice?

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#2 2013-06-11 07:45:39

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 86,636

Re: balls in an urn

Hi mom;

The first two are correct.

The third answer I am getting

15 / 28

When stuck for homework type problems. Draw a tree!

Do I divide by two for the probability that exactly one marble is yellow?

You multiply in this case because there are two. The tree will make it clear.


In mathematics, you don't understand things. You just get used to them.
Of course that result can be rigorously obtained, but who cares?
Combinatorics is Algebra and Algebra is Combinatorics.

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#3 2013-06-11 07:52:31

mom
Member
Registered: 2012-04-25
Posts: 94

Re: balls in an urn

I drew the tree. What step did I miss in the third problem?

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#4 2013-06-11 07:53:47

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 86,636

Re: balls in an urn

Hi;

Count up all nodes at the bottom that have exactly one yellow. What do you get?


In mathematics, you don't understand things. You just get used to them.
Of course that result can be rigorously obtained, but who cares?
Combinatorics is Algebra and Algebra is Combinatorics.

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#5 2013-06-11 08:01:42

mom
Member
Registered: 2012-04-25
Posts: 94

Re: balls in an urn

I have 3/7 and 2/7 for the second draw. The first was 3/8
I know that I am doing something wrong

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#6 2013-06-11 08:12:18

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 86,636

Re: balls in an urn

Hi mom;

Here is the tree the two you need are marked  by a rectangle. Please fill in the probabilities.

View Image: 2013-06-11_131011.gif

In mathematics, you don't understand things. You just get used to them.
Of course that result can be rigorously obtained, but who cares?
Combinatorics is Algebra and Algebra is Combinatorics.

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#7 2013-06-11 08:19:15

mom
Member
Registered: 2012-04-25
Posts: 94

Re: balls in an urn

that gives a better image than how they are teaching me. Thank you. I can see it now. They have us do a tree with fractions and I get confused easily.

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#8 2013-06-11 08:21:01

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 86,636

Re: balls in an urn

Did you get the answer you need?


In mathematics, you don't understand things. You just get used to them.
Of course that result can be rigorously obtained, but who cares?
Combinatorics is Algebra and Algebra is Combinatorics.

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#9 2013-06-11 08:22:04

mom
Member
Registered: 2012-04-25
Posts: 94

Re: balls in an urn

I did thanks again

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#10 2013-06-11 08:22:58

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 86,636

Re: balls in an urn

I hope it was 15 / 28.


In mathematics, you don't understand things. You just get used to them.
Of course that result can be rigorously obtained, but who cares?
Combinatorics is Algebra and Algebra is Combinatorics.

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#11 2013-06-11 10:55:47

mom
Member
Registered: 2012-04-25
Posts: 94

Re: balls in an urn

sorry was working on a group project. Yes it was 15/28

Thanks again

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#12 2013-06-11 12:36:29

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 86,636

Re: balls in an urn

Hi;

Your welcome.


In mathematics, you don't understand things. You just get used to them.
Of course that result can be rigorously obtained, but who cares?
Combinatorics is Algebra and Algebra is Combinatorics.

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