Well, depends on your definition of »mathematician« – I'd probably go for a question about the very basics of algebra. Something like: How many elements does th

Well, depends on your definition of »mathematician« – I'd probably go for a question about the very basics of algebra. Something like: How many elements does the symmetric group S_3 have? (Optionally: Write down its Cayley table.) Is it abelian? Not exactly relevant to practice, and utterly trivial, but I suppose nobody comfortable with formal math should any problem solving it on the spot.