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## #1 2013-04-16 01:05:36

Agnishom
Real Member
From: Riemann Sphere
Registered: 2011-01-29
Posts: 24,429
Website

### Marbles without replacement (probability)

A bag contains 10 red marbles, 10 green marbles, 10 yellow marbles and 10 blue marbles. You reach into the bag and grab a marble, then reach into the bag and grab a second marble. The probability that the second marble is the same color as the first marble is a/b, where a and b are positive, coprime integers. What is the value of a+b?

Note: You do not place the first marble back into the bag.

Is it 9/39?? But they tell it is wrong

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
I'm not crazy, my mother had me tested.

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## #2 2013-04-16 01:24:43

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Hi;

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #3 2013-04-16 01:28:25

Agnishom
Real Member
From: Riemann Sphere
Registered: 2011-01-29
Posts: 24,429
Website

### Re: Marbles without replacement (probability)

Why?

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
I'm not crazy, my mother had me tested.

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## #4 2013-04-16 01:37:20

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Supposing that answer were correct in some way, does it answer this question.

a/b, where a and b are positive, coprime integers

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #5 2013-04-16 01:44:59

Agnishom
Real Member
From: Riemann Sphere
Registered: 2011-01-29
Posts: 24,429
Website

### Re: Marbles without replacement (probability)

Thanks btw

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
I'm not crazy, my mother had me tested.

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## #6 2013-04-16 01:51:26

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Everything checking out now?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #7 2013-04-16 02:07:58

Agnishom
Real Member
From: Riemann Sphere
Registered: 2011-01-29
Posts: 24,429
Website

### Re: Marbles without replacement (probability)

Yes

'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
I'm not crazy, my mother had me tested.

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## #8 2013-04-16 02:13:46

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Probability and combinatorics are the toughest things in mathematics. The best of the best can often make mistakes. We are going to make more mistakes.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #9 2013-04-26 22:11:43

gAr
Member
Registered: 2011-01-09
Posts: 3,479

### Re: Marbles without replacement (probability)

An addition to the question in #1:

What's the expected number of marbles to be picked till we get two marbles of the same color?

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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## #10 2013-04-26 22:46:14

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Hi gAr;

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #11 2013-04-26 23:22:30

gAr
Member
Registered: 2011-01-09
Posts: 3,479

### Re: Marbles without replacement (probability)

Hi bobbym,

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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## #12 2013-04-26 23:30:27

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Hi;

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #13 2013-04-26 23:53:41

gAr
Member
Registered: 2011-01-09
Posts: 3,479

### Re: Marbles without replacement (probability)

Hi,

But answer must be independent of the number of marbles, isn't it?

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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## #14 2013-04-27 00:07:53

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Hi;

The draw is without replacement so the probability changes on each draw.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #15 2013-04-27 00:35:59

gAr
Member
Registered: 2011-01-09
Posts: 3,479

### Re: Marbles without replacement (probability)

That makes sense.
I think the way I thought is wrong, I first picked up 5 marbles and then calculated my answer from there.
I forgot to consider the probability of obtaining those subsets.

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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## #16 2013-04-27 00:39:16

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Hi gAr;

I have to get some sleep, see you later.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #17 2013-04-27 00:42:16

gAr
Member
Registered: 2011-01-09
Posts: 3,479

### Re: Marbles without replacement (probability)

Hi bobbym,

Okay, take rest, see you later..

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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## #18 2013-04-27 00:43:21

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Thanks and thanks for coming in with a good problem.

Did you mean post#15?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #19 2013-04-27 17:22:32

gAr
Member
Registered: 2011-01-09
Posts: 3,479

### Re: Marbles without replacement (probability)

Hi bobbym,

I tried the problem again

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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## #20 2013-04-27 17:28:56

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Hi gAr;

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Offline

## #21 2013-04-27 18:16:56

gAr
Member
Registered: 2011-01-09
Posts: 3,479

### Re: Marbles without replacement (probability)

Hi bobbym,

I think it's correct.

The addition looks good to me:

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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## #22 2013-04-27 18:38:11

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Hi gAr;

This is what I did;

The boxed part is incorrect. There is no winner for one throw!

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #23 2013-04-27 19:00:39

gAr
Member
Registered: 2011-01-09
Posts: 3,479

### Re: Marbles without replacement (probability)

Hi,

Do we need to include 1?
Because we need at least two marbles to check whether they are of the same color?

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

Offline

## #24 2013-04-27 19:02:26

bobbym
From: Bumpkinland
Registered: 2009-04-12
Posts: 107,133

### Re: Marbles without replacement (probability)

Hi gAr;

We were both posting at the same time. Check post #22.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #25 2013-04-27 19:05:47

gAr
Member
Registered: 2011-01-09
Posts: 3,479

### Re: Marbles without replacement (probability)

Yes, now it's all right!

"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

"Data! Data! Data!" he cried impatiently. "I can't make bricks without clay."

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