How to average the results of all rows for one column?

I am new to making reports, and going through a book on the subject.  However, the steps I am following for "Setting Aggregation Rules" are not working.  Simply put, I want to have the last row in my test report show an average of all values for its column.  Here is what I am doing. I created a report from the subject area "Activity History", and it has three columns. The column names in order are, Date Fiscal Mth/yr, # of Open Activities, and # of Closed Activities.  There are no filters, and when it runs, each row shows a