Elkar wrote...
If the numbers were formatted as text, then simply changing the
format might not be enough. . . .

Not 'might', isn't.

. . . Now that you've set the new format to Number, re-enter the
numbers. You can just select each cell and hit F2 then ENTER.
....

Or copy a blank cell, select the other cells (C3 and G1:G5), and run
the menu command Edit Paste Special, Add.

" wrote...
C3 has "1139.28" G1 = "400.00" G2= "280.00" G3 = "65.00"
G4 =" 206.52" G5="38.5" so therefore, G1:G5 = 990.02, . . .
....

You mean G1:G5 sum to 990.02. Does =SUM(G1:G5) return 990.02? You're
confusing the matter by displaying these cells' values in double
quotes unless you mean they're text that just happens to look like
numbers.

One more thing you need to check. Run the menu command Tools Options
to display the Options dialog. Click on the Transition tab. You may
need to uncheck Transition formula evaluation.