I am currently studying for the international olympiad for astronomy and astrophysics (the syllabus can be found here: https://www.ioaastrophysics.org/syllabus/). I am currently using Carroll Ostlie's Introduction to Modern Astronomy and Astrophysics to learn the theory.
However I need a good resource for practicing problems. I always finish the problem set at the back of the textbook, however I would like a greater volume of challenging problems to prepare me for the rigor of IOAA. I am aware that I can redo past year papers from other countries or redo some IOAA papers, but I'm saving them for practice before IOAA. So are there any good (preferably topical) resources that I can use with good, challenging problems? Thanks!