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'
database name:db_id shipping
city table city Cols: city id dtype integer, city name dtype text, state dtype text, population dtype integer, area dtype real, 
customer table customer Cols: customer id dtype integer, customer name dtype text, annual revenue dtype integer, customer type dtype text, address dtype text, city dtype text, state dtype text, zip dtype real, phone dtype text, 
driver table driver Cols: driver id dtype integer, first name dtype text, last name dtype text, address dtype text, city dtype text, state dtype text, zip code dtype integer, phone dtype text, 
truck table truck Cols: truck id dtype integer, make dtype text, model year dtype integer, 
shipment table shipment Cols: ship id dtype integer, customer id dtype integer, weight dtype real, truck id dtype integer, driver id dtype integer, city id dtype integer, ship date dtype text, 
