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Math Help - Quartiles/Normal Distribution

  1. #1
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    Quartiles/Normal Distribution

    I have a test tomorrow and I'm working on a practice problem right now that I can't see to understand. Please help!

    The amount of Jen's monthly phone bill is normally distributed with a mean of $76 and a standard deviation of $12. Find the first quartile,

    I understand that the mean goes in the middle of the bell curve, and the SD is the intervals between the other numbers on the curve. But how do I get that ?

    If it helps, the answer is $67.96.
    Last edited by mr fantastic; September 1st 2009 at 01:09 AM.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by juicysharpie View Post
    I accidentally placed this question under the other statistics section. Sorry if it confused anyone. Here we go again...

    I have a test tomorrow and I'm working on a practice problem right now that I can't see to understand. Please help!

    The amount of Jen's monthly phone bill is normally distributed with a mean of $76 and a standard deviation of $12. Find the first quartile,

    I understand that the mean goes in the middle of the bell curve, and the SD is the intervals between the other numbers on the curve. But how do I get that ?

    If it helps, the answer is $67.96.
    The first quartile is the value that is exceeded with probability 0.75, or is not exceeded with probability 0.25. You look this up in a table of the standard normal distribution to give the z-score corresponding to the quartile (~=-0.674) and convert that to a bill in the usual way.

    CB
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