Engineering Statistics
posted by Christian .
"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 ztable to go from a probability/S.D. to a zvalue to plug into X = u + S.D.*z How do I use the ztables 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

Take a zchart, 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 0Z (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 $900.842*9.5=$82. 
what numbers did u plug in for z and x
in the formula z=xu divided by the s.d
Respond to this Question
Similar Questions

statistics
A maker has the opportunity to purchase several acres of land that is divided up into several lots for building singlefamily homes. It is known that the average sale price of all of the lots is $80,000 and the standard deviation of … 
Statistics
The standard deviation of GPAs (grade point averages) of all male students at a college is .35 and the standard deviation of GPAs of all female students at the same college is .34. Suppose we take one sample of 40 male students and … 
statistics
A sample of the number of study per week of college students is collected. Some of the sample results of the survey are given in the following table Study Hr 4, 10, 20,24, 34, 36, 42 Z score 2.5,1.75,0.5,0, 1.25,1.5,2.25 P. Rank … 
math
A study has shown that 20.05% of all college textbooks have a price of $80 or higher. It is known that the standard deviation of the price is $10. What is the mean price of the college textbooks? 
STATISTICS
A random sample of stock prices per share (in dollars) is shown. Find the 90% confidence interval for the variance and standard deviation for the prices. Assume the variable is normally distributed. 
final exam
a study shown that 20% of all college textbooks cost at least $70.it is known that the standard deviation of the prices of all college textbook is $9.50.assuming that prices of all college textbooks follow a normal distribution, find … 
statistics
The following list is a sampling of the costs of various statistics textbooks used at several colleges. Give the upper confidence limit for a 95% confidence interval estimate for the mean cost of all statistics textbooks. 109.95; 92.00; … 
college statistics
the math SAT scores for all students who applied to the school of engineering are normally distributed with M=629 and standard deviation of 60. the school of engineering has decided to accept those students whose scores fall in the … 
Social Studies
1. Why do businesses seek a equilibrium price? 
Social studies
1. Why do businesses seek an equilibrium price?