Personally, I think it is very necessary to do all 660 questions and understand them. If you have 400 questions, you can do what you can. If you get one, you'd better do 400. I think 400 can look up knowledge points and play a role in finding leaks and filling gaps. The questions on 400 will pay attention to the combination of knowledge points. Although there will be deviations in the questions, you can basically go through the knowledge points that will be forgotten when you do it again.