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Standard normal distribution question!! assume that adults have IQ scores that are normally distributed with a mean of 100 and a standard deviation 20. Find P15, which is the IQ score separating the bottom 15% from the top 85%. how do i find this!
You're looking for the 15th percentile. You need to find the z-score corresponding to a proportion/probability of 0.1500, then use the z-score formula to solve for x.
This site is rather good for straight-forward notes and formulas: http://stattrek.com/probability-distributions/standard-normal.aspx?Tutorial=Stat
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Here's a Z-table if you need it: http://www.math.unb.ca/~knight/utility/NormTble.htm
Do you have a calculator with an inverse normal function?
If not, you'll need to use that z-table that I linked to. Find the P=0.1500 in the body of the table and read across to the margins to find the associated Z-score. Sometimes you have to estimate between two values and pick the closest one. follow the stattrek.com link to get the z-score formula, plug in your z, µ, and σ, and solve for x.