select u.university, qd.difficult_level, round( count(qpd.question_id) / count(distinct u.device_id), 4 ) as avg_answer_cnt from user_profile u join question_practice_detail qpd on u.device_id = qpd.device_id join question_detail qd on qpd.question_id = qd.question_id group by u.university, qd.diffi...