SOLUTION: Last year, a study showed that the average ATM cash withdrawal took 65 seconds with a standard deviation of 10 seconds. The study is to be repeated this year. How large a sample wo

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Question 154857This question is from textbook
: Last year, a study showed that the average ATM cash withdrawal took 65 seconds with a standard deviation of 10 seconds. The study is to be repeated this year. How large a sample would be needed to estimate this year's mean with 95 percent confidence and an error of ±4 seconds? This question is from textbook

Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
Assuming a normal distribution and using the old standard deviation as a measure of the standard deviation,
you can use the confidence interval formula,
z%2Asigma%2Fsqrt%28N%29=4
For 95% confidence, z=1.96.
1.96%2A10%2Fsqrt%28N%29=4
sqrt%28N%29=1.96%2A10%2F4
N=%2819.6%2F4%29%5E2
N=24.01
To be sure, we'll round up to N=25.