I think "multiple and factor" is very important, because he has a lot of knowledge points, and the quality of learning directly affects all the learning content in the future: the learning method starts with multiplication, leads to multiple and factor, and talks about prime numbers and composite numbers from factors; In multiples, learn the characteristics of multiples of 2, 3 and 5, especially multiples of 3. In addition, "using letters to represent numbers" is also very important, which is the first step to learn equations and solve practical problems.