Jane Street Assistant Trader - Analyst interview questions

I applied online and got the phone interview invitation within a few days. The online application is the simplest I've ever seen. I applied to both London and Hong Kong office and the interview is from London office. A trader interviewed me.

Interview process was relatively smooth, interviewer called right on time. After a one sentence intro, he got directly into mental math questions. Besides that, he also asked some probability questions and my confidence in each answer. All in all, not incredibly hard, but definitely not easy. Very much unlike interviews for other companies.