How do I make a formula?

[GaryDrown1gmailcom]

I would like to add numbers from different pages, how do I do that? There are a multitude of sheets:

Sheet 1 cell F21, Sheet 2 cell F20 Sheet 3 cell F20 Sheet 4 cell F20 Sheet 5 cell F20 Sheet 6  cell F20  Sheet 7  cell F20 ...

Thanks

[#2]

@GaryDrown1gmailcom You need to indicate sheet name in your formula.
Something like this: 

='Sheet 1'!F21+'Sheet 2'!F20 + .....

Please share the file, if you want me to build the entire formula