step 1
Find out the mean
we have the data set
Mean=(338.28+328.93+341.03+331.29+356.01+329.03+328.46+586.34+401.23+386.37+388.43+367.31)/12
Mean=(4,482.71)/12
Mean=373.56
step 2
Find out the sample standard deviation
Subtract the mean from each data point and square it
(338.28-373.56)^2=1244.6784
(328.93-373.56)^2=1991.8369
(341.03-373.56)^2=1058.2009
(331.29-373.56)^2=1786.7529
(356.01-373.56)^2=308.0025
(329.03-373.56)^2=1982.9209
(328.46-373.56)^2=2034.01
(586.34-373.56)^2=45275.3284
(401.23-373.56)^2=765.6289
(386.37-373.56)^2=164.0961
(388.43-373.56)^2=221.1169
(367.31-373.56)^2=39.0625
Add the squared deviations
S=56,871.6353
Divide by the number of data sets minus 1
S/(n-1)
where
n=12
56,871.6353/(12-1)=5,170.14866
Take the square root
sample standard deviation=√(5,170.14866)
sample standard deviation=71.90
Part B
Remember that
z =(x - μ)/s
where
μ=373.56
s=71.90
I take the bills