It was a typical Wednesday evening when Alex stumbled upon a cryptic message from his friend, Rachel. The note read:

Alex nodded. "Yeah, I've used it to prep for my math exams. It's a great resource."

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.

"I think I see a pattern," Alex said, his eyes scanning the screen. "The missing solutions all seem to be from the more challenging problems."

Alex's eyes widened. "What do you mean?"

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

Rachel's eyes sparkled with excitement. "That's exactly what I need your help with. I've been working on a problem set from this book, but I've encountered a strange issue. Some of the solutions have vanished!"