Help with homework problem.

[mal86_05]

The EPA range for a car is 20-28mpg, how large of a sample would be required to estimate your mean mpg with an error of +/- 1 mpg with 90% confidence

I'm not really sure where to begin and what formula to use. My teacher doesn't really explain things and the book reads like a foreign language to me.

I know I am solving for n = the sample size. The only formula I could find needed the standard dev. and I'm sure how to get that with little information. Please help, I'm ready to cry.

 

[royhaas]

I assume you are using the normal distribution, in which case you need the standard deviation. The "range" business doesn't seem to add any useful information to solve the problem. However, it could mean that \(\displaystyle E=z\sigma/\sqrt{n} = 4\). In that case, you must know the standard deviation and the EPA confidence limit.

This is a very poorly designed question.