Hello. I am preparing for the GRE testing with focus mainly in quant. I have taken the test in July and I didn't get the score that I wanted (157). I found that I have problems regarding combination and permutations and also probabilities. What do you think that I have to do and study (i.e. books and examples). Any info will be extremely helpful because I have arranged to give again the test in the beginning of November. Thank you in advance.