posted by SHA on .
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?
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 = (score-mean)/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.