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Math Help - percentiles and z scores

  1. #1
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    percentiles and z scores

    I've already missed two points on this one..but I can't really figure out what I did wrong. Help would be appreciated.

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    The scores on a standardized test are normally distributed with a mean of 50 and a standard deviation of 10. What is the percentage of scores that are greater than 75? Use the Standard Normal Table.

    my work:

    z = (x-mean)/standard dev.
    z = (75-50)/10
    z = 2.5

    then, using the standard normal table... 2.5 give you .9938.

    to percentage... 99.38%
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    Where did I go wrong? thanks.
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  2. #2
    o_O
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    What you found was p(z < 2.5), the percentage of scores that are less than 75.

    What you're looking for is: p(x > 75) = p(z > 2.5) = 1 - p(z < 1.5)
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  3. #3
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    ah... alright. So my answer would be .62%. Thanks, man.
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