i got 16.8 , but she’ll need roughly 17 gallons of paint :)
how i did it :
1. so you know she’ll need 4/5 per book case, and there are 16 bookcases.
2. So you do 16 x 5, then you get 84
3. and you divide 84 by 5. and you’ll get 16.8, that’s how many gallons of paint she needs