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i have a small statistics problem:

The time spent by students to solve an exam, following a normal distribution, has a mean of 80 minutes, and a standard deviation of 20 minutes.

.. and asks:

What's the minimum time that 30% of the students spent on it?

Can anyone help me finding the way to solve this?

Thanks

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    Yes, i read the homework section, and i guess i should had followed that guideline. I just didn't know about it. Next time i need i'm going to follow the rules. Thanks a lot people.2011-11-13

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Let $X=$the time spent by a student on the test. $X$ has normal distribution with mean 80 and standard deviation 20. You need to solve $P[X>a]=.3$ Passing to the standard normal distribution, need to solve $P[{X-80\over 20}>{a-80\over20} ]=.3,$ or, $ [Z>{a-80\over20} ]=.3 $ where $Z$ is the standard normal variable. You need to do a "reverse lookup": go to a table/calculator for the CDF of the standard normal and find the value $z$ with $P[Z>z]=.3$. Call this $z_3$. Then set $ z_3= {a-80\over20} $ and solve for $a$.