The formula window is simple but has quite a bit of functionalityBut why dig out the calculator, punch in the numbers, and type the totals into the cells when you can have Word do the work for you?
To make Word do the work, position your cursor in one of the month cells in the last row of the table and choose Table | Formula. You will get a window similar to the one shown here.
When you first open the window, the Formula box will read =SUM(ABOVE), as long as you have positioned your cursor in the bottom row of the table. If you place your cursor in one of the cells in the right-hand total column, the Formula box will instead read =SUM(LEFT). Word is smart enough to take a look at your data to figure out which formulas make the most sense.
Immediately below the Formula box, you see the Number format box. As you might expect, the options in this box allow you to indicate how you want to format the result. In the example shown here, the output would be formatted with a leading dollar sign.