Rachel explained that she had been working on a specific section, "Equations and Inequalities," but when she returned to her notes, some of the solutions to the problems were missing. She suspected that someone might have been playing a prank on her, but she needed Alex's help to figure out who and why.

"Have you ever used this book?" she asked.

Rachel's eyes lit up. "That's it! I think I know who might be behind this."

Alex's curiosity was piqued. He arrived at the library, and Rachel was already seated at a table, surrounded by stacks of math textbooks and papers. She handed him a worn-out copy of "Problems in Mathematics" by V. Govorov.