What is the average number of shipments done by the Kenworth trucks?
"Kenworth" is the make of truck; average = Divide (Count(ship_id where make = 'Kenworth'), Count(truck_id where make = 'Kenworth))
SELECT CAST(COUNT(T2.ship_id) AS REAL) / COUNT(DISTINCT T1.truck_id) FROM truck AS T1 INNER JOIN shipment AS T2 ON T1.truck_id = T2.truck_id WHERE T1.make = 'Kenworth'