MathConfused
I started off with the OG and then purchased Jeff Sackman's GMAT hacks review in hopes that it would go over the basics more.
The first error was relying on the OG for reviewing math concepts. The OG is excellent for question practice, but its review section is not even worth reading. I am very familiar with Sackmann's products. In my view, his math sets are excellent and well worth the price. His math guidebook, however, does not cater to everyone. He's very knowledgeable, but kind of assumes that his audience is too. Consequently, he breezes through each topic. I never could gain more than a superficial understanding of the concepts with his book. And for a PDF file, the book isn't cheap either.
The best advice that I can offer is to look into the Manhattan math series. Others have already mentioned this, so I will just leave it at that.