See as you said that only 20 days are left for CAT exam so in this time don't start any new topic or syllabus. You need to brush up all you basics. You need to make sure that you have a strong hold on all the basics.

Along with this in these 20 days start giving a mock test everyday and after giving test make sure you analyse the solution of each mock test. Analysing is the most important thing here. By analysing I mean you need to check whether the approach you used in the correct question is best approach or else there is another way to do it which can save your time. Also check where did you committed a mistake in the wrong questions. Was it a step mistake or the whole approach is wrong. And finally check where you got stuck in the questions which you were not able to attempt. Make sure you note down all the points so that you can use them to solve similar type of questions in future.

This analysing helps you in overcoming your mistakes and also to build up a strategy to attempt your paper which will definitely help you in improving your marks and percentile. Through this you can also figure out your strength and weakness which will help you in attempting easy questions first that will help you in reducing the chance of errors and missing an easy question and also will give you enough time to attempt medium level questions so this will overall improve your speed and accuracy leading to increase in percentile.

Remember in these last days analysing is very important without that you will never be able to improve your percentile and also make up a strategy to attempt your paper without that you wont be able to give your best.

All the best for your preparations and exams!!

Though the time is less and I am assuming that you must have made a study plan initially.

CAT does not need to mug up things instead you should have a good aptitude which requires proper strategy and knowledge of what all will come in the exam.

Now that less time is left, by now you should have clearly identified your strong areas and weak areas. If not then start with previous year papers and then tests series. Once you identify the same, you should build on your strength now and pick a few weak areas which you can work afresh from a concept level (if you have enough time then only go to the concept level)

The very important thing is test series/mock tests. You should be practising online-offline mocks, sectional-full length mocks. Also attempt 3-4 mocks in a week but do not over do.

This is not enough. Once you give the test, analyse it. Without analysing you won't get what you are falling short of.

Join similar interest groups which give daily updages on latest news, current affairs and some smart tips to prepare in this short span of time.

Also revise basic formulas and concepts which will strengthen your base.

This is what to pay attention in these 20 days.

Good Luck.