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Y2KDREAD 10-30-2004 07:49 PM

Probability Question I am stuck on
 
I know this should be a really easy question I should be able to get in two questions, but I suck at math. We are talking about combonations and permitations, so use that crap.

From among 80 light bulbs, 10 are defective. How man samples of 8 have at least 1 deffective bulb.

Can anyone help?

molloby 10-30-2004 08:20 PM

All right. X~B(8,1/8)

P(X/=0) = 1-P(X=0) = 1-(7/8)^8 = 0.656391

Therefore from 10 samples the expected number with at least one defective bulb is 6.56391 or 7.

Y2KDREAD 10-30-2004 08:27 PM

i think that is way too complicated for my calss, just checked on the profs website and she posted the answer, she told it is 19,547,186,230

molloby 10-30-2004 08:42 PM

Where is she pulling the numbers from, how may samples are we talking?

Getting 19.5 billion samples of 8 from a set of 80 is quite impressive...

Y2KDREAD 10-30-2004 08:46 PM

i have no idea, all i know is that when we figure it out in the calculator (TI-83+ for me), we are using the nCr and and nPr things. I assume this one will be nCr because order does not matter. I figured that it would be 80 nCr 10 or (C(80,10)) divided or subtracted by something that represnts the C(8,1), i just can't figure it out.

molloby 10-30-2004 09:08 PM

I think I've got it:

There are 80C8 unique ways of choosing 8 balls from 80 and 70C8 ways of choosing 8 balls from 70. So:

Number of with defects = Total number of choices - number of choices without defects

= 80C8-70C8
=19,547,718,623

Enjoy.

Y2KDREAD 10-30-2004 09:14 PM

thanks a lot, i don't really have a math sorta mind (i can do it, but it takes me awhile) but it takes me a bit to figure stuff out, and half the time i get fed up and just skip stuff.

molloby 10-30-2004 09:19 PM

Fair enough.

Honestly I'm just avoiding my own work, it is always easier to do someone else's than your own.

Y2KDREAD 10-30-2004 09:22 PM

and it gives a sense of accomplishment that you don't get from doing your own work

fckm 10-31-2004 11:30 AM

No offense, but it seems as if your teacher's not doing her job correctly. She should be teaching you how to do probability and statistics, not how to plug numbers in to you calculator. I would suggest that you go back to your textbook and reread the section that defines what permutations and combinations are. Hopefully, you'll gain a better understanding of why they are defined the way they are and how they work.


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