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#1 2010-01-27 03:03:39

katerinaaa
Member
Registered: 2010-01-27
Posts: 2

problem with probabilities

Hi all, I have a problem with probabilities but I can't solve it.
The problem is :
"I have 3 folders. A folder of them has a letter while the others are empty.
The probablity to find the letter in the folder, if we touch the top of the folder is p_j , if there is letter in this folder.
We check folder 1 and we did not touch the letter.
Which is the possibility the letter to be inside the folder 1 ??"

I don't understand,  the possibility is 1/3 again I think.

Could you help me please ??

thanks a lot

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#2 2010-01-27 14:19:07

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 82,673

Re: problem with probabilities

Hi katerinaaa;

Are asking what is the probability of choosing a folder with the letter in it if there are 3 folders and only one folder has a letter in it?


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

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#3 2010-01-28 09:24:28

mathsyperson
Moderator
Registered: 2005-06-22
Posts: 4,900

Re: problem with probabilities

Three things could have happened:

1) The letter is in folder 1, and you missed it when you checked earlier.
This has probability 1/3 * (1-p_j).

2) The letter is in folder 2. This has probability 1/3.

3) The letter is in folder 3. This also has probability 1/3.

And for completeness:
[4) The letter is in folder 1, and you found it earlier. Probability 1/3 * p_j.]

We already know the 4th option didn't happen, so the probability of the letter being in folder 1 is

or more simply,

This means that the probability of the letter being in folder 1 is:
1/3, if p_j = 0.
0, if p_j = 1.
1/5, if p_j = 1/2.
etc.


Why did the vector cross the road?
It wanted to be normal.

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#4 2010-02-07 01:46:30

katerinaaa
Member
Registered: 2010-01-27
Posts: 2

Re: problem with probabilities

thanks a lot for your help!

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#5 2010-09-06 23:26:11

leealden06
Member
Registered: 2010-09-06
Posts: 4

Re: problem with probabilities

In probability, the problem of birthday, or accession to the birthday paradox probability that a number of people chosen at random for a couple of them have the same problem birthday.The is to calculate approximately the probability that a room of people, have at least two the same birthday.

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#6 2010-09-07 08:12:26

bob bundy
Moderator
Registered: 2010-06-20
Posts: 6,258

Re: problem with probabilities

hi leealden06

I've met this before.  Are you asking how to calculate this?

Bob


You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#7 2011-02-26 18:40:38

frankMwarimbo
Member
Registered: 2011-02-26
Posts: 1

Re: problem with probabilities

whats is the difference between independent events and dependent event?

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#8 2011-02-26 19:19:03

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 82,673

Re: problem with probabilities

Hi frankMwarimbo;

Two events are independent if the occurrence of one has no effect on the probability of the other.

Please take a look here:

http://www.mathsisfun.com/definitions/i … event.html

http://www.mathsisfun.com/data/basic-co … ciple.html

http://www.learningwave.com/chapters/pr … ndent.html


In mathematics, you don't understand things. You just get used to them.
I have the result, but I do not yet know how to get it.
All physicists, and a good many quite respectable mathematicians are contemptuous about proof.

Online

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