Posted by **Arlene** on Wednesday, April 4, 2012 at 4:31pm.

The general manager for Gordon's Self Service Gasoline is interested in estimating the mean number of gallons of gasoline that are purchased by customers at their Philadelphia location. He would like his estimate to be within plus or minus 0.50 gallons, and he would like the estimate to be at a 99% confidence level. Past studies have shown that the standard deviation for purchase amount is 4.0 gallons. Find the required sample size.

- statistics -
**MathGuru**, Wednesday, April 4, 2012 at 8:36pm
Formula:

n = [(z-value * sd)/E]^2

...where n = sample size, z-value will be 2.58 using a z-table to represent the 99% confidence interval, sd = 4, E = 0.5, ^2 means squared, and * means to multiply.

Plug the values into the formula and finish the calculation. Round your answer to the next highest whole number.

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