I am trying to put together a formula to sum multiple columns of data found
on a different tab. For example, the "Data" tab has three columns with 15
rows of data. I want to sum column A, B and C; then add A and B and divide
by C.

Here is what I'm trying to work with:
=(Sum(Sum(Data!A:A)+(Sum(Data!B:B))/(Sum(Data!C:C)))

But I'm getting a different total than when I do the same calculation on the
tab with the data.

Any assistance would be appreciated!!