statistics
posted by jon .
The mean of nonresidential buildings is 30 years and the standard deviation of the ages of the buildings is 5 years. Determine the probability that a random sample of a group of 50 nonresidential buildings will have a mean age of less than 27.5 years.

Z = (scoremean)/SEm
SEm = SD/√n
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion/probability related to the Z score.