SOLUTION: Suppose we want a 90% confidence interval for the average amount spent on books by freshman in their first year at college. The amount spent has a normal distribution with standar

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Question 820418: Suppose we want a 90% confidence interval for the average amount spent on books by freshman in their first year at college. The amount spent has a normal distribution with standard deviation $20.
(a) How large should the sample be if the margin of error is to be less than $2?
Answer by stanbon(75887) About Me  (Show Source):
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Suppose we want a 90% confidence interval for the average amount spent on books by freshman in their first year at college. The amount spent has a normal distribution with standard deviation $20.
(a) How large should the sample be if the margin of error is
to be less than $2?
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Since E = z*s/sqrt(n)
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n = [z*s/E]^2
----
n = [1.645*20/2]^2 = 16.45^2 = 271 when rounded up
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Cheers,
Stan H.
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