Can you do math in a pivot table?

[SandsB]

My pivot table gives me
SystemName #ofUsers TotalDuration

So if 10 users logged into SystemA for 5 minutes each it would show me:
SystemA 10 50

and then following that it would show every other system, number of users and the total duration of those sessions:

SystemA 10 50
SystemB 9 30
SystemC 12 20
etc.

Is there a way to make the pivot table give me the average duration per system? Like
SystemA 10 5?