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## #1 2010-01-28 02:03:39

katerinaaa
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### 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

## #2 2010-01-28 13:19:07

bobbym

Online

### 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.

## #3 2010-01-29 08:24:28

mathsyperson
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### 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.

## #4 2010-02-08 00:46:30

katerinaaa
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### Re: problem with probabilities

thanks a lot for your help!

## #5 2010-09-07 21:26:11

leealden06
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### 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.

## #6 2010-09-08 06:12:26

bob bundy
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### 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

## #7 2011-02-27 17:40:38

frankMwarimbo
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### Re: problem with probabilities

whats is the difference between independent events and dependent event?

## #8 2011-02-27 18:19:03

bobbym

Online

### Re: problem with probabilities

Hi frankMwarimbo;

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

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.