@jmiller
=SUM(F5+G5) is a bit of an odd construct. Either do =Sum(F5:G5) or do =F5+G5. You don't need to wrap a Sum() around a simple addition.
dizzycat: If you want to add the value of F5 to G5 and display the result in G5, then you will need a macro.
Copy the code below, then right click the sheet tab, select "View Code" and paste the code into the code window.
cheers, teylyn