SELECT up.university, qd.difficult_level, count(qpd.question_id) / count(DISTINCT qpd.device_id) AS avg_answer_cnt FROM user_profile up, question_practice_detail qpd, question_detail qd WHERE qpd.question_id = qd.question_id AND qpd.device_id = up.device_id GROUP BY up.university, qd.difficult_level