What you want to do is add column for Task Actual Hours in your report, and then for the column, indicate to summarize by Average.
Next, apply a Grouping for your report (any Grouping that you want). And, also be sure to add any additional filters to the report that you may need. Then, when you run the report you will see the summarized Task Actual Hours indicating the Average.
Terry Hynd EBSCO Information Services