Hi pepo,
Where did you hear that 'Mixture' questions count so much? Questions that would be strictly defined as Mixture questions (re "a 20% nut mixture is combined with a 30% nut mixture to form a 26% nut mixture...." and not Ratio/Percent/Average questions) typically show up just 1-2 times on Test Day, so that question type does NOT represent a big category on the GMAT.
If you have any specific prompts that you'd like to discuss, then you should post them in the appropriate Quant Forums. Most Mixture questions are essentially just about doing the necessary algebra/arithmetic, but you can sometimes use Tactics (TEST VALUES or TEST THE ANSWERS) to answer those types of questions.
GMAT assassins aren't born, they're made,
Rich