If the cell is not empty delete its contents. To do this click inside the last cell in the fourth column click on Layout in the ribbon and then click on Formula at the far right. The Formula dialog box appears as shown.
You may notice in this table of contents that there are sub-levels.
To add the formula in the Word table do the following. Insert Table Formulas in Word Instructions. This will display a Formula Dialog Box which will suggest a default formula which is SUM LEFT in our case. Learn how to work with Function in a Words Table.