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Alex and Billie have to empty all of the bookshelves in the library, and move the books into storage.
Alex, on her own, can empty the bookshelves in 18 hours.
Billie, on her own, can also empty the bookshelves in 18 hours.
Unfortunately, Drew likes to move the books from storage back onto the bookshelves, and she can refill the bookshelves in 36 hours.
How long will it take the three of them, working at the same time, to empty the bookshelves?