Default tables allow you to continuously add rows when you fill out the form, therefore there isn't a fixed number of rows when the template is created. This doesn't allow you to have a row for the average at the bottom of the same table. The only way to do it is to create a second table for the average and cross-reference it to the first table.
To start, navigate your cursor to the left tab and choose the 'Default table' under 'Table'. Input your data.
Next, add a 'Prefilled' table under that and set one cell as your display cell.
To average a column in a default table, use the formula to the display cell:
=AVERAGE(Source_Table!Column:Column)
Where Number1 is the start of the range, and Number2 is the end of the range.
Average calculation includes Number1, Number2, and the numbers in between.
For example:
Given that
Table1 is the Source_Table
A is the Column
Table2 cell B1 is your display cell
Use the formula to Table 2 Cell B1,
=AVERAGE(Table1!A:A)