Posted by SHA on Saturday, November 19, 2011 at 8:57pm.
An accounting firm is planning for the next tax preparation season. From last year's returns, the firm collects a systematic random sample of 100 filings. The 100 filings showed an average preparation time of 90 minutes with a standard deviation of 140 minutes. What is the probability that the mean completion time is between 1 and 2 hours, i.e., 60 and 120 minutes?

statistics  PsyDAG, Sunday, November 20, 2011 at 4:02am
It would help if you proofread your questions before you posted them.
You probably have a typo. With SD = 140, one SD below the mean (90) would be 50. This means that you do not have a normal distribution.
However, if the distribution is normal,
Z = (scoremean)/SD
Insert the correct values to find Z.
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportions related to your Z scores.
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