Probability help
posted by Martin .
The heights of the buildings are normally distributed. The mean of the heights is 275 feet and the standard deviation is 30 feet.
A. What is the probability that the buildings height is over 280 feet?
B. What is the probability that a sample of 80 buildings is gathered and the mean height of the buildings was over 280 feet?

a. http://davidmlane.com/hyperstat/z_table.html
b. sample standard deviation= sqrt(30^2/80)= 3.35 recompute, given mean 275, sigma 3.35