one or more of the cells F5,G5 or H5 probably have text in them, either
a number entered/pasted as text, or a "blank" made by pressing the space
bar (and inserting a space character).
For the first problem, copy an empty cell, select F5:H5 and choose
Edit/Paste Special, selecting the Values and Add radio buttons, then
click OK. This will coerce any "text" numbers to real numbers.
For the latter, use the del key or Edit/Clear/Contents rather than the
spacebar to clear the cell.
Note that SUM(F5:G5,-H5) will ignore text, including space characters
(but it won't flag "text numbers").
