...
To display the average value for any specific column, click the ellipsis ( ). For reference, to display the average value for the Tax column, click the ellipsis.
The ellipsis menu for each column becomes active, enabling the Edit column dialog box to open only after the Sum-Up row is inserted.
...
The Edit column dialog opens. Click the Sum-up type drop-down and select the required type. For reference, Avg is selected.
The Tax column displays the Avg value. Likewise, you can choose the Sum-up type for each column as required.
...