What is the average duration of songs that have mp3 format and resolution below 800?
return songs ;return formats of  #1 ;return #1 where #2 is  mp3 ;return resolutions of  #1 ;return #1 where #4 is  lower than 800 ;return #1 of  both #3 and  #5 ;return durations of  #6 ;return average of  #7
['select', 'project', 'comparative', 'project', 'comparative', 'intersection', 'project', 'aggregate']