Just waiting for the results of my mid-term and would like to thank some members of this forum for their helpful direction! Stuck on a problem though...thought I had a reasonable grasp of how to calculate intervals when the level of confidence is provided but this problem asks for the level of confidence:

A random check of 81 records of inmates from all federal prisons found the confidence interval for mean prison terms per prisoner to be 2.13 years to 2.287 years. Assuming a standard deviation of 0.4 year, what is the level of confidence provided by this interval?

If I'm correct, the mean would be 2.2 and I think I am to use the formula xbar +/- (confidence interval) x s/square root of n; any direction would be greatly appreciated as that is as far as I've been able to get...

Thanks very much...Shannon