Question

a study of 40 professors showed they spent on average 13.6 minutes correcting a student papers. Given a standard deviation of 17, the 90% confidence interval of the mean time for all the test papers is what?
Formula:
CI90 = mean + or - 1.645(sd/√n)

With your data:
CI90 = 13.6 + or - 1.645(17/√40)

I'll let you take it from here.

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