There is no full-proof way you can get into any college. It’s truly a little bit random in combination with what you did in high school. My biggest advice is to show your personality/passions and create a narrative with your application. Brown in particular really emphasizes students who want to go outside of their major to learn things with their Open Curriculum. So, really there isn’t a certain extra curricular or volunteer job that you can get that will get you accepted. You just have to show that your passion (in this case math) is shown through your activities and that you go above and beyond when it comes to your passions. Notice I’m saying passion a lot. I just got into the Brown PLME program yesterday which I was not expecting. But, looking back I showed in my application that in addition to my interest in health care and science, I really cared about community service and music. I had examples in my high school resume that accounted for these three things. This must’ve shown the admissions officers that I would use Brown’s facilities to study science and music while participating in their community. That’s what I mean when you have to create a narrative. Good luck in your future endeavors and remember you will end up where you are supposed to be!