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 the mean price of all college textbooks.

Answer 1

http://www.jiskha.com/display.cgi?id=1369182329

Put in this applet the standard deviation, then the mean as zero.

Then start at -6, check the area. EVentually, you will get an area of .20

Then, say -6.5 is the point, you know that is really 70, so add 70 +6.5, and you have the mean.