Standard Deviation of Officials in Student Services Salaries

If 45 officials in student services earned an average of x(with a line over it) = $50,340 each year:

a. Assume that the standard deviation is 16,920 fro salaries of officials . Find the 90% confidence interval for the population mean salary of such personal. What is the margin of error.
b. If SD is 10,780, Find the 90% confidence interval and margin of error?
c.If SD is 4830.00, find the 90% confidence interval and margin of error?
d. Compare the lengths of confidence intervals in a-c, As the sd decreases does the length of the 90% confidence level decrease?