To be clear, these are hard contest questions that most people, including older kids and adults, probably can't answer them without proper learning and practice

To be clear, these are hard contest questions that most people, including older kids and adults, probably can't answer them without proper learning and practice. I have not checked them out in detail, but it appears that https://artofproblemsolving.com/ https://artofproblemsolving.com/ is a good resource. They also have interesting online and physical book series: https://artofproblemsolving.com/store https://artofproblemsolving.com/store (I have no affiliation.)