Someone I know who got accepted to Stanford recommends the guy who tutored hin:
Saïd Boufatis, Math Professor who prepares for GMAT, GRE y SAT,
he can be contacted by mail frbsaid@gmail. com or by cel 15-5-505-1192.
I just took the GMAT this August and I used the Princeton Review "Cracking the GMAT" book to prepare. I found the math review very straightforward and useful for re-learning the math that I had forgotten from high school and college. Also, the GMAC people publish a book with practice problems. I found that helpful because beyond learning the math, I needed practice drills to get used to doing them quickly and guessing when I didn't know the right answer. I ordered both off Amazon. Hope this is helpful.