Getting average

Hi,

I have an XML file in the following format:

<cutomers>
	<group income=1000>
		<person age=10/>
		<person age=15/>
		<person age=30/>
	</group>

	<group income=2000>
		<person age=10/>
		<person age=40/>
	</group>

	<group income=5000>
		<person age=20/>
		<person age=20/>
	</group>
</customers>

I would like to find the average of incomes for people of a particular
age? I am using XSLT 1.0. A double pass is not practical. Maybe this is
stupid simple and I am missing something basic, but I would appreciate
some help on this.

Thanks,
HC.