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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 -

    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.

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