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May 7th, 2018, 07:44 PM   #1
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Unhappy 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?
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May 7th, 2018, 08:19 PM   #2
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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 $\Phi^{-1}(0.99) = \dfrac{t-2.0}{0.5}$
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May 7th, 2018, 09:41 PM   #3
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how come a) and c) have red font with yellow background and say "undefined control sequence \dfrac ?
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May 7th, 2018, 10:16 PM   #4
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Quote:
Originally Posted by Alex87 View Post
how come a) and c) have red font with yellow background and say "undefined control sequence \dfrac ?
looks ok to me. Maybe you can't contact the LaTex rendering server?
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May 8th, 2018, 08:09 AM   #5
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maybe..... ahhh this is all to confusing for me lol
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May 8th, 2018, 08:29 AM   #6
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Quote:
Originally Posted by Alex87 View Post
maybe..... ahhh this is all to confusing for me lol
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) = (t-2)/(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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