hello ramakant......

it is not easy to prepare jee mains in 20 days but in the remaining few days you can prepare as the above instructions

select the easy subject from all the three next complete the syllabus how much you can by concentrating on the important topics and also formulas so that you can solve any problem easily and next remaining two subjects also in same way but in these subjects you have to concentrate on the formulas and the previous question papers and also try to solve minimum 150-200 mcqs daily without fail

i hope this information is helpful

all the best.

Now, you don't have enough time to prepare for JEE Main,you are advised to prepare atleast the main topics. I'm providing you a list of some important chapters for JEE Main, here it goes:

Mathematics:

General Things about Parabola, Ellipse, Hyperbola

Basics of Limits (for ex.: Lt((Sin x)/x) for x -> 0 is 1)

Integration Formulas

Binomial Expansions

Permutation And Combination

Probability

Inverse Trigonometric Functions

Chemistry

Chemical Kinetics

Chemical Bonding

Metallurgy

s block elements

Basic Nomenclature

Grignard Reagent

SN1, SN2 Reactions (Reactivity, Stability Orders of Organic Compounds)

Carboxylic Acids and Their Reactions

Biomolecules & Polymers

Physics

Basics of Kinematics

Rigid Body Dynamics (+ Centre of Mass)

Current Electricity (+ Capacitance)

Geometrical Optics

Modern Physics

Logic Gates (for JEE Mains especially)

For further information, kindly ask through comment section.