To learn mathematics, we must first have a correct method, step by step, from the shallow to the deep, first understand the basic concepts and lay a good foundation. If you don't lay a good foundation in front, you won't be able to learn in the back. Addition, subtraction, multiplication and division are the most basic calculation methods in mathematics. The first thing to understand is addition. The inverse operation of addition is subtraction. When several or several identical numbers are added, we use multiplication, and vice versa. No matter what kind of operation method, start with one digit, master the method, then learn two digits, three digits until multiple digits.