Auto Calculate the total in Repeating Table

"Total" displays sum of all the values in the field "Duration", for all the rows in the repeating table.

Select the control that will display the total value, right click and select the "Text Box Properties".

Set the "Default Value" of the field, click on the "fx" button.

Type "sum(fieldname)" in the formula window, where "fieldname" is "EstDuration" in this case.

Make sure "Refresh value when formula is recalculated" checkbox is selected.