Question 1169033
population mean is 21.60
population standard deviation is 8.80
sample size is 64 customers.
probability sample mean is less than 20.00 is what?


standard error = standard deviation / square root of sample size.
standard error = 8.80/sqrt(64) = 8.80/8 = 1.10


z-score = (x-m)/s
x is the raw score
m is the mean
s is the standard error.


in this problem, .....
z-score = (20 - 21.60) / 1.10
solve for z-score to get:
z = -1.454545.....
round to -1.45


area to the left of a z-score of -1.45 is be equal to .0735.
that's the probability that the sample mean will be less than 20.00.