What is the average number of cars on trains that run in the east direction?
calculation = DIVIDE(count(id), count(train_id))
SELECT CAST(COUNT(T1.id) AS REAL) / COUNT(DISTINCT T1.train_id) FROM cars AS T1 INNER JOIN trains AS T2 ON T1.train_id = T2.id WHERE T2.direction = 'east'
database name:db_id trains
cars table cars Cols: id dtype integer, train id dtype integer, position dtype integer, shape dtype text, length dtype text, sides dtype text, roof dtype text, wheels dtype integer, load_shape dtype text, load number dtype integer, 
trains table trains Cols: id dtype integer, direction dtype text, 
