Why I like this problem: This problem is a great review of basic, sometimes long-forgotten concepts. Although the knowledge required dates back to elementary grades, students can develop strategies to make the solution more efficient. It also requires some thinking outside the box and perseverance for the last two questions. I provided students with the 100 grid below to help the visual learners.
Observations from class: I was surprised to see that students struggled with the concept of factors and multiples despite being in high school. They often confused the two and had to remind each other of the definitions several times throughout the class period. I gave students a 100 grid to help them organize their thoughts and work with number patterns.
Follow-up questions I asked:
1. What number facts can you use to make the process of elimination more efficient?
2. How can you make the process of finding the numbers systematic?
Required skills / content: Knowledge of factors and multiples.