Final answer:
Percy and Annabeth stay on the 600th floor of the Lotus Hotel and Casino in the book 'Percy Jackson and the Olympians: The Lightning Thief'.
Step-by-step explanation:
In the book 'Percy Jackson and the Olympians: The Lightning Thief', Percy and Annabeth stay on the 600th floor of the Lotus Hotel and Casino. The Lotus Hotel and Casino is a magical establishment that can trap people inside, making them lose track of time. Percy and Annabeth end up staying there for several days without realizing it.