Question 1041533: A study of nickels showed that the the standard deviation of the weight of nickels is 200 milligrams. A coin counter manufacturer wishes to find the 90% confidence interval for the average weight of a nickel. How many nickels does he need to weigh to obtain an average accurate to within 20 milligrams?
A. 100
B. 164
C. 273
D. 385
Answer by Boreal(15235)   (Show Source):
You can put this solution on YOUR website! The standard error has to be 20 mg
It is z*s/sqrt (n), since we don't have n, we can't use a t-test. Also, the number is likely to be quite large.
20=1.645*200/sqrt(n)
multiply both sides by (sqrt(n)); divide both by 20
sqrt(n)=200*1.645/10=32.9
n=32.9^2=1082.41 or 1083 nickels, rounding upward
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