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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: 
May 24th, 2011, 03:43 PM  #4  
Senior Member Joined: Mar 2011 Posts: 105 Thanks: 0  Re: probability question ????? Quote:
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  
Senior Member Joined: Mar 2011 Posts: 105 Thanks: 0  Re: probability question ????? Quote:
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  
May 24th, 2011, 07:28 PM  #6 
Newbie Joined: May 2011 Posts: 13 Thanks: 0  Re: probability question ?????
daivinhtran, You are missing the whole point of this question. Let's look at a more familiar problem. Suppose you toss a fair coin 30 times. You would expect that 50% of the tosses will be heads for a total of 15 heads. But in reality you might only toss 13 heads or toss 22 heads. In fact, you can toss either 0 heads or 1 head or 2 heads or ,,, even 30 heads. Now we can talk about the probability that you toss 0 heads or the probability that you toss 1 head ,..., the probability that you toss 30 heads. Using your logic you can not toss a coin 7 times since you have to get 3 1/2 heads. But of course you can toss a coin 7 times. What you are computing with 6.6 and my example of 15 is what is called the expected value or the average. If you surveyed many groups of 12 customers you would find out that the average or expected value will be close to 6.6. http://www.mathondvds.com http://www.mathtutor1.com 

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