If you really are at the sub-500 level - I'd recommend that you first get some help so that you are at least guided in the right direction.
From there, then you can perhaps self-prep as best you can and then decide whether you need further assistance or whether you're good to go.
Some of the concepts in this video may be too advanced for you at the moment, but the beginning portions can help you understand the scope of probability: http://www.gmatpill.com/problemsolving/ ... -part1.mp4
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