Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion/probability related to the Z score. Percentile rank is the proportion below a particular score.
OR…
http://davidmlane.com/hyperstat/z_table.html
the average score on the statistics quiz was a 70, and there was a standard deviation of 10 points. If the z-socre was -1.0 what is the percentile rank?
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