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#1 2007-06-02 00:17:08

Daniel123
Member
Registered: 2007-05-23
Posts: 663

Probability

I don't get this.... can someone please explain.

If there is n beads in a bad, and 6 of them are black, then I can understand that the probability of removing a black one is 

. If the rest of the balls in the bag are white, then I can understand that the probability of removing a white ball is 
. But if you take a white ball out, without replacement, will there be
balls left in the bag, and why? I just can't visualise what is going on.

Thanks.

Last edited by Daniel123 (2007-06-02 00:40:08)

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#2 2007-06-02 01:03:17

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

Re: Probability

It'll be

.

Before, there were n-6 white balls out of n balls altogether. If you remove a white ball then both of those counts decreases by 1. So there are (n-1) balls, (n-7) of which are white.

You can see that is has to be this way by thinking about what would happen if you had removed 6 white balls without replacement instead of just the one.


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

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#3 2007-06-02 01:34:14

Daniel123
Member
Registered: 2007-05-23
Posts: 663

Re: Probability

Right ok I get it now. Thanks big_smile

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