select
university
,difficult_level
,round(count(qpd.question_id)/count(distinct up.device_id), 4) 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
order by
university, difficult_level