Help calculating 95% confidence interval?

I have this lab report due and I was given instructions as to how to get Excel to do my calculations for me. However, I’ve hit a snag and am hoping someone can help.

On a few of my observation sets, the standard deviation is 0. This seems to be tripping up Excel and I get a “number error”. I have the command entered correctly, with the correct cell number.

So.. how can I get Excel to calculate this, or how do I calculate it myself? I took Stats, but years ago, and I find myself really overwhelmed with the instructions posted on the web.

Thanks in advance.