Beginning of Stats
posted by Laddi .
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.