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

from user_profile as up

 join question_practice_detail as qpd

on up.device_id = qpd.device_id

 join question_detail as qd

on qpd.question_id= qd.question_id

group by university,difficult_level

;