The Professor's Response
Hi Professor, From #42 of the 7.4 HW, the degrees of freedom is 127 and that number is not on the t-table so I don't know how to get the answer appropriately.
please help, thank you.
See the professor's answer below.
In that section, you are trying to distinguish between when to use a z-critical value or a t-critical value. Remember, we are using the simplified rule that states if n > 30, we will find a z-critical value, otherwise we will find a t-critical value.
In the problem you asked about, since n = 128, we need a z-critical value, so you do not need to find degrees of freedom to find the 1.282. All you need to do is to find alpha/2 and go to the t-table. The answer you need will appear in the last row of the table.
Hope that answers your question,
Professor McGuckian