SOLUTION: A study of nickels showed that the standard deviation of the weight of nickels is 150 milligrams. A coin counter manufacturer wishes to find the 80% confidence interval for the ave

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Question 1086483: A study of nickels showed that the standard deviation of the weight of nickels is 150 milligrams. A coin counter manufacturer wishes to find the 80% confidence interval for the average weight of a nickel. How many nickels does he need to weigh to obtain an average accurate to within 10 milligrams?

a. 225

b. 369

c. 613

d. 159

Answer by jim_thompson5910(35256) (Show Source):
You can put this solution on YOUR website!
We will use this formula
n = ( (z*sigma)/E )^2

The variables are
n = sample size
z = critical value
sigma = population standard deviation
E = margin of error
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Use a table or calculator to find that
Critical value = z = 1.28 (at 80% confidence)

In this case, we're given
sigma = 150
E = 10

Plug those values into the formula to get:
n = ( (z*sigma)/E )^2
n = ( (1.28*150)/10 )^2
n = ( 192/10 )^2
n = ( 19.2 )^2
n = 368.64
n = 369 ... round up to the nearest whole number

Final answer is choice B) 369