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  1. #1
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    finding percentile




    HI

    can someone help me set this up to solve. i know i had missed this on one my previous exam, and trying to study for tomorrow final exam.

    scores on a test are normally distributed with a mean of 65.3 and a standard deviation of 10.3. find p 81 (percentile 81) which separates the bottom 81% from the top 19%,

    the answer is 74.4


    how do i sovle for the percentile.

  2. #2
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    Let X follows N (mean=65.3 and sd 10.3).

    Now you have to calculate K such that P(X <K )=.81

    Convert into Standard Normal
    P[ (X-mean)/sd < (K-mean)/sd] =.81
    P[Z <(K-mean)/sd] =.81

    Now from normal table calculate Za such that P[Z<Za]=.81

    then solve K.
    In the long run, we're all dead.

  3. #3
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    still a little lost

    sorry but

    so far i got

    6.3398< kmean = 834.30

    then I am not so sure what to do next

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    See these examples in the link .. it may help.
    http://cnx.org/content/m16977/latest/
    In the long run, we're all dead.

  5. #5
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    First find Znormal value that corresponds to area from -infinity to +Z as 0.81 (given).
    Now this value of Z = (X-Mean)/ standard deviation.Since mean & s.d. are known you can find X which is unknown.

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