I participated in some high-school-level math competitions, but I never invested much of my time in them at that time, something that I regret immensely.
Now that I am an engineering grad student, I want to do something about it and grab the opportunities to compete in college-level math competitions. But, I am having a hard time finding free materials to prepare for them, online. I found a lot of high-school-level resources, but haven't had much luck in finding college-level resources.
I know I'm already asking for a lot of help, but I'm poor in a country where the dollar is really expensive. So if possible (I know it might not be possible), please help me find only free resources for preparation.