A 5-line solution that a 5th grader can understand, but a 9th grader might miss. This is the heart of Russian math.
These platforms host comprehensive collections of past ARMO problems, often translated into English: russian math olympiad problems and solutions pdf
in PDF. This is a foundational text containing 320 unconventional problems from Moscow State University competitions. Art of Problem Solving (AoPS) : Offers printable PDF collections of the All-Russian Olympiad A 5-line solution that a 5th grader can