Posted by Emmanuel on Monday, November 15, 2010 at 9:34pm.
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?

statistics  MathGuru, Monday, November 15, 2010 at 9:41pm
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.