The number of non-valuable files Brennan downloaded in the first round is 70/100*800 = <<70/100*800=560>>560 files.
The number of valuable files Brennan downloaded in the first round is 800-560 = <<800-560=240>>240
When he downloaded 400 new files, there were 3/5*400= <<3/5*400=240>>240 non-useful files, which he deleted again.
The total number of valuable files he downloaded in the second round is 400-240 = <<400-240=160>>160
To write his research, Brennan had 160+240 = <<160+240=400>>400 useful files to reference to write his research.
#### 400