Crystal Reports Average formula not working

Message

   i am working on a report that is grouped by:

Name
Invoice. Date (grouped in month)

i.e.
John           total job completed 300 <--that's a distinct count.
8/2009                                      250<--that's a distinct count.
9/2009                                        50<--that's a distinct count.

I would like to calculate the average for the month....
how would i do that?