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February 3rd, 2013, 06:30 PM  #1 
Member Joined: Apr 2010 Posts: 33 Thanks: 0  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 bellshaped and symmetric) 
February 3rd, 2013, 09:45 PM  #2 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,912 Thanks: 1110 Math Focus: Elementary mathematics and beyond  Re: Probability and Statistics Problem 
February 4th, 2013, 08:23 AM  #3 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,912 Thanks: 1110 Math Focus: Elementary mathematics and beyond  Re: Probability and Statistics Problem
With as the zscore, as standard deviation and as the mean, we have A zscore of 1 gives 84.13%, so the number of cellphone users who use the phone less than 40 minutes per day is 0.8413 * 4000 ? 3365. 

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