900 x 3/4 = <<900*3/4=675>>675 people came before the start of the concert.
900 - 675 = <<900-675=225>>225 people did not come before the start of the concert.
225 x 5/9 = <<225*5/9=125>>125 people came few minutes after the first song.
So, 125 + 80 = <<125+80=205>>205 people came to the concert that was not able to be at the start of the concert.
Therefore, 225 - 205 = <<225-205=20>>20 people did not go to the concert.
#### 20