It took almost one week after the initial email, I interviewed with the HR first, then met the mentor whom I was going to work with. The questions that the HR asked were pretty straightforward, some behavior questions including how you deal with the conflicts with the other group members. The interview with the mentor included an excel test, which is pretty simple since the mentor would guide you whenever you got a question. I got the offer at the end of that week, and it's really a positive interview experience.