Hi redfield,
The example that you're using emphasizes the 'math work' that you have to do in many Quant questions, so if your math skills are a bit rusty, then you'll certainly want to hone those skills. To score at a high level in the Quant section, you'll need more than just math skills though - the Quant section of the GMAT is based heavily on Tactics, patterns, logic, Number Properties, etc., so those other areas also have to be accounted for.
For free math practice and help, I recommend that you set up an account at Khan Academy (
www.khanacademy.org). The site is completely free and makes the learning a bit more fun and 'game-like' (as opposed to the dry academic approach taken by most books). While the site is vast, you should limit your studies to basic Arithmetic, Algebra and Geometry. After spending a little time re-building those skills, you can start your GMAT studies.
1) How have you been scoring on your practice CATs (including the Quant and Verbal Scaled Scores)?
2) What is your goal score?
GMAT assassins aren't born, they're made,
Rich