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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.
Some cause happiness wherever they go; others, whenever they go.
If you can not overcome with talent...overcome with effort.

## #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
Novice

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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.
Some cause happiness wherever they go; others, whenever they go.
If you can not overcome with talent...overcome with effort.