posted by Emmanuel .
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?
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.