Making math more engaging for students is a challenge that every teacher faces on a daily basis. These authors write that they are constantly searching for rich problem-solving tasks that cover the necessary content, develop critical-thinking skills, and engage student interest. The Mystery Fraction activity provided here focuses on a key number theory concept of divisibility and connects it to many important concepts in middle school mathematics, including place value, fraction sense, scale factors, reasoning and proof, and counting. The main focus of this activity is to explore possible strategies to determine a mystery fraction that must satisfy certain conditions and is equivalent to a given number.