What function would you use to find the mean in microsoft excel document

Hey there! Hello!

I have a pretty current version of Microsoft Office, so it might be different depending on which version you're using. But in the case of Office for Mac 2016, this is what you do:

Go to the "Formulas" tab. 
"Average" should be under your Recently Used drop–down, but if it's not, it'll be in the AutoSum drop–down.
Type your numbers you want to calculate, if you haven't already. 
Below the numbers (or wherever in your spreadsheet), press "Average," and you'll see the option to select which cells to average out. (Attachment 1). 
After you select them, you should see your averaged results! (Attachment 2). 

Hope this helped you out! Feel free to ask me any additional questions you may have. :-)

Igot the answer of 15.8 years.