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July 22, 2014

July 22, 2014

Posted by **Christian** on Tuesday, October 11, 2011 at 3:13pm.

I have S.D. = 9.5, P = 20% = .2, and I don't know where to go from here. I know the answer is $82 based on a calculator online, but I can't find out how to use the z-table to go from a probability/S.D. to a z-value to plug into X = u + S.D.*z How do I use the z-tables when given a percentage that is outside the bounds of the table? Mine only goes from .1 to .0005, so there are no listed values for .2

- Engineering Statistics -
**MathMate**, Tuesday, October 11, 2011 at 5:14pmTake a z-chart, for example:

http://www.math.unb.ca/~knight/utility/NormTble.htm

20% of all books are $90 or higher, which means 80% of all books are $90 or lower.

Look up the chart for 0-Z (first chart in the above link) for 0.800. You'll find it on the row 0.8, and between columns 0.04 and 0.05 which gives approx. 0.842 SD from the mean after interpolation.

The mean price is therefore $90-0.842*9.5=$82.

- Engineering Statistics -
**bob**, Thursday, November 15, 2012 at 11:58pmwhat numbers did u plug in for z and x

in the formula z=x-u divided by the s.d

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