Celine was charged $0.50 * 20 = $<<0.50*20=10>>10 for the book she returned to the library after 20 days.
If the other two books stayed at her house until the end of May, she paid 31*$0.50= $<<31*0.5=15.5>>15.5 for each book.
For the two books that stayed at her house until the end of May, Celine paid $15.5*2 = $<<15.5*2=31>>31
For the three books, Celine paid $31+10 = $<<31+10=41>>41
#### 41