Yea - I wasn't entirely sure what you meant by the "FizzBuzz" for math point. You were looking for something with more of an abstract focus?
You can have the person then turn it into a general case. What is the sum of 1...N given that the numbers divisible by x1...xk. Even that's pretty simple though.
It's simple to figure out with pen and paper but the goal is to just confirm a basic level of competence.