The avg()-function returns the average of its argument. This function requires an XPath query as argument. The query must specify an attribute that has a numeric type.
avg(//Sales.Order/TotalPrice)
This query returns the average total price of all placed orders.
avg(//Sales.Order[Sales.Customer_Order/Sales.Customer = 'Jansen']/TotalPrice)
This query returns the average total price of all orders placed by a customer named Jansen.