How to improve your quant from Q78 to Q85?

• You Mastered most of the required skills
• You Know the structure and requirements of gmat math questions
• You Understand what is tested on the GMAT

• "Silly mistakes" i.e. things that in the pressure of the gmat you do wrong although you know and understand them
• Get yourself into unnecessary complications in solving questions that can be solved much faster (not being able to see shortcuts)
• Timing issues... there are some things that just take you too much time to get right
• Mistakes that are a result of a subtlety of concepts that you were not sure about (which made you both spend time, eventually guessing or answering without confidence, sometime getting it wrong)

The Most Common Prep Mistakes:

• Concentrating on advanced concepts and hard questions. the ROI of this kind of prep is small (it would be higher if you were to advance from 49 to 51 for example). Otherwise, you are learning how to play the game at the highest level but actually never get there.
• Jumping into the tests. I see a lot of GMAT Club members jump into the CAT's and not only use up many of them (to only end up with few resources by the end) but also discourage and distract themselves. Think of it as trying to reduce the fever with a thermometer. No matter how many times you check your temperature, it will be about the same and it won't help to keep checking.
• Not timing your practice (e.g. books, mobile apps, anything you solve, you should be timing yourself)
• Study mostly from the OG. OG is not a guidebook! It is a question collection. It is not designed to ace the test but rather give you an idea what to expect. The value comes in honing your ear and practice AFTER you have covered the basics and material, not before.
• Not keeping track of their mistakes - if you encounter a mistake don't hide it and try to forget about it.
• Stretching their prep time to 6+ months. The brain is a tricky thing and the older you are, the faster it forgets things. You need to refresh and go back frequently if you have been studying over 4 months. Otherwise, it is 2 steps forward and 3 steps back.

Recommendations for Improvement:

• Perfect your basic techniques. for example: solving regular linear, one variable equations should be performed with no mistakes at all, and should take you no more than 20-30 seconds; doing simple arithmetic - again... no mistakes are allowed, do it fast and with confidence... etc...
  
• Try to understand types of math questions frequently appearing in the GMAT exam - here is GMAT Focus Syllabus
  
• Boost your confidence. My suggestion - do questions again and again, until you can do, in time, without guessing and providing full answers, until you can do a full test (21 questions of various difficulty) with no more then 2-4 mistakes.
  
• Try to look for the simplest way of solving questions. Don't "buy" complex explanations. for 95% of gmat questions there are simple explanations. Also, don't sufficient yourself with just one way to solve questions. For many questions there are more than one good way to solve. familiarizing yourself with all of them will help you find the simplest way to do it. Follow Bunuel's posts - he often provides very eye opening solutions that are quick and effective.
• Look for specialized material that concentrates (correctly) on subtleties of concepts, especially the concepts you feel are difficult for you. Usually it is really hard to come by specialized sets, but on GMAT Club forum, you can search questions by Tags and by Difficulty. You can use this forum very wisely.
• Try GMAT Club tests - these are hard math problems designed to make you work hard for every question
• TTP does Quant prep really well and their course has been highly recommended by many impressive reviewers
• Save the questions you miss. Print them out and go back to in a few days or weeks to make sure you can solve them. If you can't answer the same question again, then try again for the third time (remember - time yourself!). If you made a mistake the third time - memorize the question, the solution, and the answer. Yes, memorize and be able to write it out when needed from memory.
• Get better at solving simple things and memorize some of the shortcuts. Here are arithmetic shortcuts to remember.
• If nothing else helps, take a look at some of the timing strategies on GMAT Club
• Use GMAT Club Math book - it is a free PDF download with the most common and hardest to grasp concepts
• I would recommend focussing on accuracy in the beginning. Do the last 50 OG questions, 10 at a time after thoroughly understanding the concepts. Spend as much time as possible to ensure you get all right. for me, it took almost 2 and a half hours to complete. Once you find, you are getting 9 out of 10 right every time, you can move to practicing on a timed test, and try finishing 37 questions in 60 minutes. ( Keep accuracy at the top of your head, if you have to guess, try to eliminate at least 3 wrong answers.), but move on. This balance between speed and accuracy is the key to a higher quant score. - by Ashaker

If you Are not Quite to Q78 Yet:

• Spend more time reviewing concepts that are not clear to you (there are certainly some if you score less than 75).
• For under 70 you should probably continue review basic skills before moving forward.... take a look at Manhattan Math foundation books - they are super cheap (though no for Focus edition, so just skip Geometry)
• Try studying on the go when you have time.
• You may want to pick up a copy of the MGMAT's Roadmap with some tips for general studying (not quant specific but helpful)
• See the thread: How to Improve your Quant from Q65 to Q75