Are all the sheets configured the same ... meaning ... are the total cells
the same on all sheets (i.e. A25)?
If so, try this:
=SUM(Sheet1:Sheet15!A25)
The sheets to be included in this formula *must* be physically sandwiched
between Sheet1 and Sheet15 in the scroll bar.
You can move sheets in and out of the sandwich, changing the total.
This works well when evaluating "what if" scenarios.
Some people insert 2 blank "dummy" sheets, named "Start" and "End", and
place the actual data sheets between them.
This makes the formula:
=SUM(Start:End!A25)