20 Hours a week is good enough time if you sincerely spend that time for GMAT. It sounds like your Math is weak but you did not comment on your verbal abilities. But let me tell you even if your verbal is strong, you rather than ignoring it, capitalize on the same. A good strategy will be to devote 50 hours for the maths and 30 hours for the verbal section.
For the Maths, I will suggest you finish each of the OG problems and Quant review problems. After this purchase the
GMATClub tests (whole set of 25) and finish as many as possible. The GMATClub problems are perhaps now closest to the actual GMAT problems. If you can ace these, then you can ace the GMAT. There is good explanations provided for each of the questions. Do not spend your time on any other resource. Even this much is sufficient.
For the verbal section, purchase the Manhattan CR guide. I am telling this because, with the purchase, you will get 6 free online exams and these exams are really good. You can get other books from your friend (specially the SC book). Follow these books, taking each chapter at a time, then doing the OG problems at end of each chapter. Once you are done with this guide, at the same time you will be done with whole OG as well.
After this finish as many 1000SC, 1000CR, and 1000RC. Oh just remembered, Manhattan has one free full length exam, so you can take that as diagnostics to assess yourself initially.
Take GMATPrep exam when you feel you are almost. If you score 700+, then you are ready for the day. Repeat GMATPrep as many times as possible. Although you will get repeat questions, there will be many difficult new problems as well (because you will get your repeat rigth so not repeated are going to be harder level problems).
Best of luck.