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