select
    university,
    difficult_level,
    round(count(qpd.question_id) / count(distinct qpd.device_id),4) as avg_answer_cnt
from question_practice_detail as qpd

join user_profile as up
on qpd.device_id=up.device_id

join question_detail as qd
on qpd.question_id=qd.question_id

group by university,difficult_level