The first thing I'd do is make a list of all the questions you've already received, and make sure you can generate good answers to them. I've gotten repeat ques

The first thing I'd do is make a list of all the questions you've already received, and make sure you can generate good answers to them. I've gotten repeat questions, and variations on the same ideas, so it's a fairly straightforward way to do better. Since you have a combinatorics background, you might also consider thinking of a model that will tell you your rough odds of passing a single interview. My thinking was roughly: "I usually get 7 questions, need to get 6 correct, and I have a 70% chance of producing a decent answer. What are my odds of passing?". That can give you an idea of how much you need to improve to have a decent odds.