I think your reasons for going to b-school approach of targeting just top-tier schools make sense.
Re: the GMAT, there's no magic number, but I would aim for at least a 650 overall. Hopefully you can get at least 75th percentile or better on the Quant section. Don't stress over your lack of math/quant overall. If yo udo well on the GMAT and your GPA solid (Bs are fine), you're okay. But taking a college-level class can benefit you in another way... It can demonstrate the fact that you're serious about getting into b-school. So, definitely consider it. Don't worry about it being at a fancy school; any will do.
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