The pre-interview preparation taught us to ask clarifying questions, and in doing so I learned the calculator was one of those simple dollar-store ones with a solar panel (was told during some interviews the "calculator" ended up being the Windows calculator on computers if you didn't ask any questions). Kept coming up with different ways to test the functionality of the calculator and asking any more questions about it (like who was the target audience for the calculator, etc.) until I was stopped by the interviewer.