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#1
02-20-2007, 12:28 PM
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Statistics problems help
I have a few easy statistics problems if anyone wants to help me out.
1. Bowl has 5 chips 1 black 4 red. Two chips selected without replacement Find the prob. that the first chip is black or the second chip is black. 2. Preg. follow normal dist. mean=266 st.=16 find prob. preg. lasts more than 279 days. Thanks if someone can help me. 
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#2
02-20-2007, 12:43 PM
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Re: Statistics problems help
[ QUOTE ]
1. Bowl has 5 chips 1 black 4 red. Two chips selected without replacement Find the prob. that the first chip is black or the second chip is black. [/ QUOTE ] 1 - P(both red) = 1 - (4/5)*(3/4) = 2/5 or 40%. OR P(first black) + P(first red and 2nd black) = 1/5 + (4/5)*(1/4) = 2/5 or 40%. OR C(1,1)*C(4,2) / C(5,2) = 2/5 or 40%. [ QUOTE ] 2. Preg. follow normal dist. mean=266 st.=16 find prob. preg. lasts more than 279 days. [/ QUOTE ] P( > 13/16 SDs above mean) = 1 - NORMSDIST(13/16) =~ 20.83%.
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#3
02-20-2007, 12:58 PM
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Re: Statistics problems help
I am confused about number 1. The way I see it it should be .45, because if I draw a black on the first (1/5) + the odds that I draw a black on the second which is (1/4) since I picked a red on the first.
Thanks for the help
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#4
02-20-2007, 01:17 PM
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Re: Statistics problems help
[ QUOTE ]
I am confused about number 1. The way I see it it should be .45, because if I draw a black on the first (1/5) + the odds that I draw a black on the second which is (1/4) since I picked a red on the first. [/ QUOTE ] Actually, before you draw any chips, the probability that the 1st will be black is the same as the probability that the 2nd will be black, and that is 1/5, so you would add 1/5 + 1/5 = 2/5. Adding these works in this case because you can't get a black on both draws, i.e., they are mutually exclusive. Don't do this in cases where they are not mutually exclusive. You are using the 1/4 probability that the 2nd is black given that the 1st is red, and if you use that, you have to then multiply this by the 4/5 probability that the 1st is red as I did in my 2nd method. Be sure to understand this concept thoroughly as it is a key to your entire understanding of probability theory.
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