What is the average number of professional or master/undergraduate courses being taught by each professor?
professional or master/undergraduate courses refers to courseLevel = 'Level_500'; average number = divide(count(taughtBy.course_id), count(taughtBy.p_id))
SELECT CAST(COUNT(T1.course_id) AS REAL) / COUNT(DISTINCT T2.p_id) FROM course AS T1 INNER JOIN taughtBy AS T2 ON T1.course_id = T2.course_id WHERE T1.courseLevel = 'Level_500'