My Math Forum probability question ?????

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 May 24th, 2011, 06:23 AM #1 Senior Member   Joined: Mar 2011 Posts: 105 Thanks: 0 probability question ????? A restaurant found out 55% customers have coffees with their meal... If there are 12 people, what is the probablity to have exactly 7 people have coffee with their meal... A. 0.015 B. 0.222 C. 0.4242 D. 0.512... I don't know how to do it. But with this multiple choice: I chose D. Because I follow 55%.... So, if there are 12 people.... with 55% have coffees with their meal....... It would be 6.6 So it's between 6 and 7..... But it's closer to 7 than 6 ( it's 6.6) So it should be greater than 0.5 and I chose D Can somebody tell me how to solve it, please?
 May 24th, 2011, 07:53 AM #2 Senior Member   Joined: Sep 2009 Posts: 106 Thanks: 0 Re: probability question ????? I understand it like this, the prob is 0% because. if you have 12 ppl & 55% have cofee, then 6,6 ppl have coffee. But question says 7 people must have coffee. This can not happen.
 May 24th, 2011, 07:54 AM #3 Senior Member     Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 521 Math Focus: Calculus/ODEs Re: probability question ????? You need to use the formula for binomial probability: $P(x)=\binom{12}{7}(0.55)^7(1-0.55)^5\approx0.222$
May 24th, 2011, 03:43 PM   #4
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Re: probability question ?????

Quote:
 Originally Posted by peaceofmind I understand it like this, the prob is 0% because. if you have 12 ppl & 55% have cofee, then 6,6 ppl have coffee. But question says 7 people must have coffee. This can not happen.
you understand it, Why it can't happen?

I know it's 6,6...but question say 7 people in 12 people..... So that's why we have to find the probability to get 7 people!!!

May 24th, 2011, 03:45 PM   #5
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Re: probability question ?????

Quote:
 Originally Posted by MarkFL You need to use the formula for binomial probability: $P(x)=\binom{12}{7}(0.55)^7(1-0.55)^5\approx0.222$

THank you, Mr. MarkFL, again.....

My teacher didn't teach me this lesson.... But he put it on my Final Exam.......and didn't omit it..
I thought it was about simple lessons.
I failed that one question!!!!

Thank you, Mr. MarkFL

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