Write SQL query to solve given problem: What is the average number of professional or master/undergraduate courses being taught by each professor?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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'