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

- Engineering Statistics -
**MathMate**, Tuesday, October 11, 2011 at 5:14pm
Take 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 11:58pm
what numbers did u plug in for z and x

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

## Answer this Question

## Related Questions

- final exam - a study shown that 20% of all college textbooks cost at least $70....
- math - A study has shown that 20.05% of all college textbooks have a price of $...
- statistics - A sample of the number of study per week of college students is ...
- statistics - The following list is a sampling of the costs of various statistics...
- college statistics - the math SAT scores for all students who applied to the ...
- Statistics - The standard deviation of GPAs (grade point averages) of all male ...
- statistics - Describe whether the following study would most likely be “...
- statistics - If the average number of textbooks is 16, the standard deviation is...
- algebra - A textbook store sold a combined total of 205history and math ...
- statistics - if the average number of textbooks in professors' offices is 16, ...

More Related Questions