What are the chapters that I should concentrate more on, so that I can get at least 40 questions right in the JEE Mains?

First go through the whole syllabus of JEE and start your preparation with the topics which have more weightage. If we talk about physics then the important chapters are: Modern physics, Heat and Thermodynamics, Optics, Current electricity, Electrostatics, Magnetics. If you study these chapters properly then you will be able to attempt around 19 questions from physics section. In Chemistry inorganic part contain more weightage than organic and physical part. Read NCERT more for inorganic part. These are the most important topics of chemistry: Transition Elements and Coordinate Geometry, Periodic table and Representative Elements, Thermodynamics and Gaseous state, Atomic Structure, Chemical bonding, Chemical and Ionic equilibrium, Solid state and surface chemistry, Nuclear Chemistry and Environment. You can expect around 18 questions from these topics. For math important chapters are: Coordinate Geometry, Limits, Continuity, and Differentiability, Integral Calculus, Complex Numbers AND Quadratic Equations, Matrices and Determinants, Statistics and Probability, 3D Geometry, Vector Algebra. You can expect around 20 questions from these topics. You can connect with IIT JEE toppers they can guide you properly what would be the right way of preparation for JEE mains.