The number of students who scored 100% is 2/5*80 = <<2/5*80=32>>32
Out of the 80 students, 80-32 = <<80-32=48>>48 did not score 100%.
If 50% of the students who did not score 100% managed to score over 80%, then 50/100*48 =<<50/100*48=24>>24 students scored over 80%
The number of students who failed is 48-24 = <<48-24=24>>24
#### 24