What have you done with this problem? What is the null hypthesis and the alternative one? Do you know how to start the problem? How to get z here? Is it a two tailed test or a one tailed? Can I see a little of your work?

]]>a-Use a level of 0.05 to test the hypothesis that the mean is less than 19.

b-find the 95% one sample confidence interval for the mean.