In addition to Artistry of Education, I have a blog called Math Spies that is dedicated to math problem-solving. I post a new problem each Wednesday.
In Episode 8, students use what they know about area to determine if a building has a secret room. Click here for "How to Find Secret Rooms." In the building at the top, the hallway extends the length of the building. The building underneath has a "secret room." Students are given the dimensions of the building and the dimensions of the known rooms and they must determine if there is a secret room and how big it is.
You can find the other seven problems in the series by clicking here, and scrolling halfway down the page.
I expanded this set into a 32 task card set which you can purchase at Teachers Pay Teachers and Teachers Notebook.