Kristina: I don't know. I'm guessing it's an extremely low number.
cos:
1. I'd say the undergraduate mathematics at Brown is not as hardcore as it is in MIT/Harvard. We definitely don't have something like Math 55 at Harvard, aka the most difficult undergraduate mathematics course in the country. But, I personally know many mathematics major that use this as a window of opportunity to take graduate level mathematics class. (Every now and then there'll be a kid who has done abstract algebra + analysis out of high school).
2. Rate of students that go to Mathematics graduate school? What's the population we're talking here? The whole student body? Out of only Mathematics concentrator? (Plus, shouldn't you be asking the success rate of graduate school placement?
)
3. No comment. Seriously.