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December 4th, 2008, 08:52 AM   #1
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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?
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December 4th, 2008, 03:49 PM   #2
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Re: Simple probability question, how can I possibly be wrong?

Quote:
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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