Hi jfranciscocuencag,
To start, "mixture" questions are a relatively rare category on the GMAT - you'll likely see just one mixture question on Test Day - so you shouldn't necessarily put a lot of effort into this one question type. By extension, unless you're already doing really well on the major categories in the Quant section (such as Number Properties, Algebra, DS overall, etc.), you're probably leaving a lot of points behind that have nothing to do with mixtures. That having been said, many mixture questions are ultimately just 'math' questions (although in certain variations, you can occasionally you can use a Tactic such as TESTing VALUES or TESTing THE ANSWERS), so if you are comfortable doing Algebra then you can train to handle just about any mixture question.
1) What study materials have you used so far?
2) How have you scored on EACH of your CATs (including the Quant and Verbal Scaled Scores for EACH)?
3) What is your goal score?
4) When are you planning to take the GMAT?
GMAT assassins aren't born, they're made,
Rich