Louie got 100% - 80% = 20% of the total items incorrect, which is equal to 5 items.
So 1% is equal to 5/20 = 1/4 of an item.
Hence, the total number of items, represented by 100%, on the math quiz was 1/4 of an item x 100 = <<1/4*100=25>>25.
#### 25