Average a returned value in a report

I have a report that returns a value named Efficiency. It is based on a
query that takes the total parts produced divided by the hours charged and
returns the parts per hour. Then it divides it by the number of parts per
hour estimated and returns an efficiency value in %. This has criteria that
limit the dates in the report. The report returns the count of items and the
efficiency of the different jobs. I want to total the efficiencies in the
report and divide it by the count to get an average efficiency. How do I do
this? I am using Access 2007. Thanks for any help.