With only 20 days for cat what should I do to boost my score

As around a month time is left for CAT exam so you need to quickly brush up with your basics and start giving lots of online mock test and analyse their solutions which will help you in overcoming your mistakes and come up with a strategy to attempt your paper which will definitely help you in improving your marks and percentile. It will also help you in knowing 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 which will automatically give you enough time to attempt medium level questions so this will overall improve your speed and accuracy leading to increase in marks and percentile.
Focus on weak areas where scores are negative in mocks and make sure if there is better approach to it to finalise strategy whether to attempt such questions or leave them in order to save time.
Revise your basics , formulae and keep believing jn your strengths so you can attempt such questions more by cherrypicking them.