Good question. The sub-master roll is a paper roll made according to the model of the master roll, which can be used as an introductory exercise in the early stage of the senior high school entrance examination. It is better to focus on real questions in the later stage. The propositional law of real questions is most directly reflected in the real questions of previous years. Experts are good at grasping the shortcomings of students' learning according to their inertial thinking and habit of doing problems, thus highlighting the methods of doing problems for excellent students.