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.

# Answer

400
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Brennan was researching his school project and had to download files from the internet to his computer to use for reference. After downloading 800 files, he deleted 70% of them because they were not helpful. He downloaded 400 more files but again realized that 3/5 of them were irrelevant. How many valuable files was he left with after deleting the unrelated files he downloaded in the second round?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
400
Brennan was researching his school project and had to download files from the internet to his computer to use for reference. After downloading 800 files, he deleted 70% of them because they were not helpful. He downloaded 400 more files but again realized that 3/5 of them were irrelevant. How many valuable files was he left with after deleting the unrelated files he downloaded in the second round?
train