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May 7th, 2018, 07:44 PM  #1 
Newbie Joined: May 2018 From: Australia Posts: 3 Thanks: 0  A question I am stuck on
Can anyone please help me with this and how I work it out ? Suppose that the mean download time for a commercial tax preparation site is 2.0 seconds. Suppose that the download time is normally distributed, with a standard deviation of 0.5 second. What is the probability that a download time is: (a) above 1.8 seconds? (b) between 1.5 and 2.5 seconds? (c) 99% of the download times are slower (a higher number of seconds taken to download) than how many seconds? 
May 7th, 2018, 08:19 PM  #2 
Senior Member Joined: Sep 2015 From: USA Posts: 1,976 Thanks: 1026 
this is all pretty straightforward let $\Phi(x)$ be the CDF of the standard normal distribution a) $P[t > 1.8] = 1  \Phi\left(\dfrac{1.8  2.0}{0.5}\right)$ I leave it to you to do the arithmetic and table/software lookup. b) find $P[t < 2.5]  P[t < 1.5]$ c) find $t$ such that $\Phi^{1}(0.99) = \dfrac{t2.0}{0.5}$ 
May 7th, 2018, 09:41 PM  #3 
Newbie Joined: May 2018 From: Australia Posts: 3 Thanks: 0 
how come a) and c) have red font with yellow background and say "undefined control sequence \dfrac ?

May 7th, 2018, 10:16 PM  #4 
Senior Member Joined: Sep 2015 From: USA Posts: 1,976 Thanks: 1026  
May 8th, 2018, 08:09 AM  #5 
Newbie Joined: May 2018 From: Australia Posts: 3 Thanks: 0 
maybe..... ahhh this is all to confusing for me lol

May 8th, 2018, 08:29 AM  #6 
Senior Member Joined: Sep 2015 From: USA Posts: 1,976 Thanks: 1026  you still can't see it? b) Find P[t < 2.5]  P[t < 1.5] c) Find t such that Phi^{1}(0.99) = (t2)/(0.5) This site works much better if your pc can render the LaTex. No one else is having trouble with it as far as I know. 

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