I want to prepare for cat exam for year 2021 but my knowledge in mathematics is zero even 4 th standard students would know more than me I don't know how to cover it up even I went for many mathemati classes to but I'm not able to understand and not even clear what to study for exam please help

If by Mathematics section, you mean Quantitative Ability, it is tad tough. But they say that the only way to master Mathematics is to practise Mathematics. Quantitative Aptitude covers almost 30% of the CAT question paper. QA can be categorised into Arithmetic, Algebra, Geometry and Advanced Mathematics. Practise chapter by chapter and develop mental calculations. Like practise more of basic summation, subtraction, fractions and tables. Numbers, Percentage, Profit and Loss, Simple and Compound Interest, Ratio, Proportionality, Time-Distance, Time-Work, Mixtures, Averages, Partnership, Permutation and Combinations, Probability, Set Theory, Progression and Series, Line, Angles, Triangles, Quadrilaterals, Polygon, Circles, Ares, Volume, Height and Distance, Coordinate Geometry, Basic Trigonometry are few important topics.

Preparation tips:

Because this section involves mathematics and takes most of your time, your focus should be on solving the problems efficiently in the beginning. Even if it takes time, practise upto perfection. Practise as many questions as you can in the conventional way. As and when you get a grip on the subject, you can go for short cuts. Memorise tables, square roots, cube roots, conversion from percent to fraction, etc. Solve sample papers on Mathematics. In the beginning, do not worry about the time it takes. As you practise, you will get the hang of the subject and it will be easier for you.

Try these books:

CAT Books For Quantitative Ability:
1.NCERT mathematics books (from 6th to 10th class) ::
These books are a good way to start if one needs to clear their basics and concepts. NCERT books are best to start your CAT Preparation with only if you have a lot of time before your exam date and need to brush up on concepts before solving advanced problems.

2.How to Prepare for Quantitative Aptitude for the CAT (by Arun Sharma)
(publisher: Tata McGraw Hill)
This is a very popular book among CAT aspirants and really easy and simple to understand. This book also focuses on the basics first and has different levels of difficulties step by step.

3.Quantitative Aptitude for Competitive Examinations (by Abhijit Guha)
This book is quite comprehensive for various competitive exams, for both MBA aspirants and for those seeking jobs who need to prepare for the aptitude tests.
4.Quantitative Aptitude Quantum CAT by Sarvesh Verma
It is an extensive book for students aspiring to go to IIMs. It has a very simple, lucid and tailor made content revolving around CAT.

5.Quantitative Aptitude for CAT (by Nishit Sinha)
This book by Pearson focuses on in depth understanding of core topics for basic and advanced applications.