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Hi Emily,

You probably got 1.28 from the z table, which makes sense since you are looking for a z value; however, in order to ensure that you get the most precise answer available, you should use the t table to find this value.

Whenever the t table can be used, you should use it, since it provides an extra decimal place. It is also easier to use than the z table. The confidence levels that can be found on the t table (in order, going left to right) are: 99%, 98%, 95%, 90%, and 80%. If you go to the t table with alpha in one tail, you will find the value 1.282. Remember, that at the top of the t table, you can look up the alpha in one tail. In this case, that is 20%/2 = 10% = 0.10. If we go to that column of the table and then go to the last entry in that column, we find our critical z value, 1.282. 

See the video below to see how this is done in general: