Question

A study of 50 mathematics professors showed that they spend, on average,12.4 minutes correcting a student's final examinations. Find a 90% confidence interval of the mean time for all mathematics peofessors spend on conrrecting final examinations when standard deviation = 2.2 minutes.
CI90 = mean ± 1.645 (sd/√n)

With your data:
CI90 = 12.4 ± 1.645 (2.2/√50)

I'll let you finish the calculation.

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