I have a spreadsheet that does a very simple calculation Column M minus
Coulmn O. Column M and N are merged, Columns O and P are merged, and the
answer is supposed to appear in Column Q (which is merged with R). I have a
workbook that has 3 tabs containing this calculation and it works very well
(10 - 1 = 9).

I have another workbook with 40+ tabs containing this calculation and it
doesn't compute. I have put the same calculation elsewhere in the spreadsheet
and it works. I have also tried re-doing the calculation in the spreadsheet
in the same place and it doesn't work. Why has it all of the sudden stopped
working in it's original places.