Rick,
The real pros here may have a much simpler/elegant solution. But, FWIW, try
the following:
Copy the cell B45 in Sheet1 to the desired cell (say, A15) in Sheet3 but use
the Paste Special function under Edit and click on Paste Link. In the
formula field for A15, in Sheet3 add a plus (+) sign. Repeat these steps
copying C76 from Sheet 2 to Sheet3, A15.
After selecting C76 in Sheet2 and clicking on COPY, I didn't have to access
the Edit/Paste Special/Paste Link function for the second entry; it already
appeared in Sheet3, A15. Simply hit ENTER and the sum of the two cells in
Sheet1 and Sheet2 appeared. Any changes you make to B45 in Sheet1 and/or C76
in Sheet2 will recalculate to the new sum in A15 of Sheet3. This works for
what I understand you're trying to do.
I did the foregoing in Excel 2000. Whether the version of Excel you're
using works the same way, I can't predict. But it might be worth a try.
Don't hesitate to ask any follow up questions.
Regards,
PJF