you probably need a complete overhaul to get up to your desired score - so back to basics on everything and I guess moreso on DS. You need Official guide, MGMAT books
if you dont have them already. They help a lot on the basics and most people here uses them. Make an error log
to track what type of questions you get wrong. To get to 650 you are looking at both scores in the mid to high 30s OR a lopsided ~50 and a ~30 score. Suggested study time is 3 months but it is doable in a month - all dependent on how much information you absorb.
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