What is the average song duration for the songs that are in mp3 format and whose resolution below 800?
SELECT avg(T1.duration) FROM files AS T1 JOIN song AS T2 ON T1.f_id  =  T2.f_id WHERE T1.formats  =  "mp3" AND T2.resolution   <  800
SELECT avg(T1.duration) FROM files AS T1 JOIN song AS T2 ON T1.f_id  =  T2.f_id WHERE T1.formats  =  "mp3" AND T2.resolution   <  800
[{'avg(T1.duration)': 3.0}]