Averaging on a list with multiple undetermined entries per week

There are a couple of different approaches to this.  The most basic would be =AVERAGEIF.  This is exactly what it's for.  Another option, and the most automatic would be a pivot table.  I'm attaching a demo with both options.  on Sheet 1, after you add the data it Table1, (Columns A and B), and then add a new line in Table2, (Columns D and E), and manually add the week number, the average will display next to it.  if you add the data, and then press Ctrl+Alt+F5, and view Sheet2, you will see the pivot table and how it updates automatically.