What critical value for z would be used to test the hypotheses: H_0:μ≥15 versus H_a:μ<15, for  = 0.01 ?

What critical value for z would be used to test the hypotheses:   versus , for a = 0.01 ?

The director of an advertising agency is concerned with the effectiveness of a television commercial.

- What null hypothesis is she testing if she makes a correct decision when she says that the commercial is not effective?