If you see the formula, then format the cell as General (or anything but text)
and reenter that formula.
If you see the wrong answer, then maybe you have calculation set to manual.
Tools|Option|calculation tab
is how you'd change it in xl2003 menus.
If that's not it, then maybe your values in those cells (C5:C8) aren't really
numbers. Reformat that range as General (not text) and reenter the values.