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April 25, 2015

Homework Help: Engineering Statistics

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

"A study has shown that 20% of all college textbooks have a price of $90 or higher. It is known that the standard deviation of the prices of all college textbooks is $9.50. Suppose the prices of all college textbooks have a normal distribution. What is the mean price of all college textbooks?"

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

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