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 December 4th, 2008, 08:52 AM #1 Newbie   Joined: Sep 2008 Posts: 9 Thanks: 0 Simple probability question, how can I possibly be wrong? This question drives me nuts. Please help. In a certain population 55% are smokers and 20% don't drink coffee. It is known that 50% are smokers who drink coffee. From this population, one person is chosen at random: The probability that the selected person drinks coffee and doesn't smoke? A. 0.15 B. 0.3 C. 0.36 D. 0.45 My answer is 25% (or 0.25), basically it's 55%+20% = 75% Then 100%-75%= 25% But it doesn't show up. How can I possibly be wrong?
December 4th, 2008, 03:49 PM   #2
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Re: Simple probability question, how can I possibly be wrong?

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 Originally Posted by Majestic_Q This question drives me nuts. Please help. In a certain population 55% are smokers and 20% don't drink coffee. It is known that 50% are smokers who drink coffee. From this population, one person is chosen at random: The probability that the selected person drinks coffee and doesn't smoke? A. 0.15 B. 0.3 C. 0.36 D. 0.45 My answer is 25% (or 0.25), basically it's 55%+20% = 75% Then 100%-75%= 25% But it doesn't show up. How can I possibly be wrong?
There are 4 possibilities for each person. I'll call smoker S and coffee drinker C, with ' indicating not.
You are given P(C&S) = .5, P(S) = .55, P(C') = .2. Therefore P(C)= .8, and P(S'&C) = P(C)-P(C&S) = .3

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