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February 3rd, 2013, 06:30 PM   #1
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Probability and Statistics Problem

In a survey of 4000 cell phone users, we obtained that the mean time of cell phone usage is 35 minutes per day with a standard deviation of 5 minutes.

Find the number of people (among total 4000) who will use a cell phone less than 40 minutes per day. (Assuming the distribution is bell-shaped and symmetric)
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February 3rd, 2013, 09:45 PM   #2
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Re: Probability and Statistics Problem



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February 4th, 2013, 08:23 AM   #3
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Re: Probability and Statistics Problem

With as the z-score, as standard deviation and as the mean, we have





A z-score of 1 gives 84.13%, so the number of cell-phone users who use the phone less than 40 minutes per day is 0.8413 * 4000 ? 3365.
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