First the whole, then the order. (Whole hundred, whole ten, last order) That is to say, first calculate what can add up to exactly = 100, then calculate exactly = 10, and finally calculate the remaining orders. Three-digit addition example1:5+3+7; First add 3+7 to get the integer 10, then add +5= 15, and the three-digit number is reduced continuously. Example 2: 28-3-8; With 28-8=20 and then -3= 17, an example of three-digit mixed operation is 3: 21-7+9; First use 21+9 = 30;

Again-7 = 23; Actually, there are many examples, such as: 1 1+33+27=? For questions like this, first scan the numbers with your eyes, then add up the numbers of 10 with your mouth, then add up the single digits that can get the whole 10, and finally add up the remaining single digits, and you can read them directly. 10、40、60、7 1=7 1

Such as: 123+260+377=? First, scan the numbers with your eyes, then add up the hundreds of digits with your mouth, then add up the ten digits that can get the whole hundred, and finally add a single digit, and you can read the results directly. 100、300、600、760=760。

By exploring the sequence of addition and subtraction operations and the process of addition and subtraction mixed operations, I realized analogical thinking. Master the operation sequence of addition, subtraction and mixed operation of addition and subtraction, and can calculate correctly. In the process of solving problems, experience the connection between mathematics and life, further develop problem-solving strategies and enhance the consciousness of applying mathematics.