Questions on Algebra: Probability and statistics answered by real tutors!

Algebra ->  Algebra -> Questions on Algebra: Probability and statistics answered by real tutors!     (Log On)
Ad: Algebra Solved!™: algebra software that solves YOUR algebra homework problems with step-by-step help!


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
: 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?
Answer by Fombitz(1740) 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*sigma/sqrt(N)=4
For 95% confidence, z=1.96.
1.96*10/sqrt(N)=4
sqrt(N)=1.96*10/4
N=(19.6/4)^2
N=24.01
To be sure, we'll round up to N=25.