I have several books of standard exercises that were asked in competitive exams in France (typically, ENS/Polytechnique), from my earlier years.
In particular, a lot of them are quite interesting, either due to the techniques used or the ideas necessary to solve them. (They mostly are about sequences, series, limits, asymptotics.) In that sense, having them on Math.SE would be (I feel) a nice thing for students who want to self-study and practice.
While it seems unethical to just copy-paste them along with the solution (in CW), as the authors of the books did come up with solutions themselves, I am wondering if it is within the scope of the website to ask the questions, wait for answers (in order to have several approaches, hopefully), and eventually maybe provide my own solution (not verbatim from the books).
Note that while the answers are due to the books' authors, the questions themselves are not -- they are from previous exams, and were generally reported by the students who had to solve them.