It sounds like the cell is formatted as text. Use Format | Cell |
Number (tab) and change it to General. Double-click in the cell (as if
to edit it) then press <enter.

Incidentally, you can reduce typing by changing your formula to:

= G40 & G41

It means the same thing.

If this doesn't cure it, you probably have the calculation mode set to
manual. Press F9 to re-calculate at any time. To change to automatic
calculation go to Tools | Options | Calculation (tab) and select
automatic.

Hope this helps.

Pete