What is the average number of students who registered for the courses with a difficulty of 4?
diff refers to difficulty; DIVIDE(COUNT(student_id where diff = 4), COUNT(course_id where diff = 4));
SELECT CAST(COUNT(T1.student_id) AS REAL) / COUNT(DISTINCT T2.course_id) FROM registration AS T1 INNER JOIN course AS T2 ON T1.course_id = T2.course_id WHERE T2.diff = 4