Each book has 8,000 words because 100 x 80 = <<100*80=8000>>8,000
She can finish each book in 200 minutes because 8,000 / 40 = <<8000/40=200>>200
She will be reading for 1,200 minutes because 20 x 60 = <<20*60=1200>>1,200
She needs to check out 6 books because 1,200 / 200 = <<6=6>>6
#### 6