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