# The Delivery Mystery

Lena runs a small bookstore in Brooklyn. Every Friday, she receives a new shipment of books.  
Last Friday, she received 40 books in total — 14 were mystery novels, 10 were science fiction,  
and the remaining 16 were poetry collections.  
That evening, she sold 23 books: 8 mysteries, 7 sci-fi, and 8 poetry books. Her friend Max, who  
visited the store at 6:30 PM, noticed that one poetry book was missing from the shelf but wasn't  
recorded as sold.  
By Saturday morning, Lena did an inventory and found only 18 books left on the shelf, even  
though she expected 19. She checked the camera footage from 7:15 PM, which revealed that a  
customer had accidentally placed a poetry book among the used books pile — mystery solved!