SOLUTION: I need help with this question, thank you in advance!!! A study has shown that 20% of all college textbooks cost at least $70. It is known that the standard deviation of the pr

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Question 871483: I need help with this question, thank you in advance!!!
A study has shown that 20% of all college textbooks cost at least $70. It is known that the standard deviation of the prices of the college textbook is $9.50. Assuming that prices of all college textbooks follows a normal distribution; find the mean price of all college textbooks.
Found 2 solutions by ewatrrr, rothauserc:
Answer by ewatrrr(24785) About Me  (Show Source):
You can put this solution on YOUR website!


Hi

invNorm(.80) = .8416  Note:  80% cost < $70

.8416 = (70-m)/9.5

9.5(.8416) = 70 - m

    m = 70 - 9.5(.8416) = 70 - 7.995 = $62

Answer by rothauserc(4718) About Me  (Show Source):
You can put this solution on YOUR website!
We are given Pr(X<70) = 0.20 or 20%
consult z tables for z value associated with 0.20
z value is -0.84
we know that the z value is (X - mean of x) / standard deviation
therefore
-0.84 = (70 - mean) / 9.50
70 - mean = -0.84 * 9.50 = -7.98
-mean = -7.98 -70
mean = $77.98