select t1.university,t3.difficult_level,
       round(count(1)/count(distinct t1.device_id),4) as avg_answer_cnt
from user_profile t1
join question_practice_detail t2 on t1.device_id = t2.device_id
join question_detail t3 on t2.question_id = t3.question_id
group by university,difficult_level