When calculating 20 minus 15, we first consider that the single digit 0 minus 5 is not enough, so we divide 20 into 15 and 5. First, we subtract 15 from 20, and then subtract 15 from 15 to get 0. Then, we add 0 to the remaining five digits to get 5.

Decimal method: a mathematical calculation method, that is, when the number of digits is not reduced enough, subtract the subtraction with 10 and add the remainder to the number of digits, that is, decimal method.

Extended data

The main points of primary school mathematics learning are as follows

1, stimulate interest in learning: children are not interested in mathematics, naturally unwilling to take the initiative to understand and master knowledge points, and may be afraid of difficulties. Most knowledge points of primary school mathematics are closely related to life. By guiding children to connect mathematics with life and understanding mathematics with simple examples in daily life, it can have a positive incentive effect.

2. Cultivate study habits: The computing ability cultivated in primary schools will always affect children's science study in junior high school, high school and even university. Therefore, it is necessary to cultivate children's good computing ability during primary school, and require children to be able to calculate quickly and accurately when facing more complicated operations. Children's computing level can be improved by rewarding a certain number of questions within a limited time.

At the same time, this method can cultivate children's habit of carefully examining and doing problems.

3. Improve the level of Olympic Mathematics: Olympic Mathematics can greatly improve children's thinking level. At the same time, some Olympic mathematics contents may directly promote the understanding of some knowledge points in junior and senior high schools. The Olympic Mathematics is difficult to understand, so you can buy corresponding Olympic Mathematics exercises for repeated training, such as "Olympic Mathematics Course" and "Draw inferences from others".

In addition, each chapter of classic exercises should be digested and understood repeatedly, and the classic solutions should be understood through basic exercises. After repeated training, the level will be improved, so that more complicated problems can be solved.

4. Strengthen daily training: Mathematics learning needs perseverance. Whether it is the knowledge points in the class or the knowledge points of the Olympics, it is difficult to succeed only by backrest formula. All the knowledge you have learned must be consolidated and improved through corresponding exercises. Just keep a proper amount of daily exercise, too much or too little will have no corresponding effect.

5. Get through the knowledge network: Most of Xiaoshengchu's math problems are not simple representations of classic problems, but the superposition of various knowledge points and strong thinking. Therefore, the structure of knowledge network is particularly important. And this must be based on the premise that every knowledge point has been fully mastered. Training on weekdays, especially the final sprint training, requires a lot of practice of real questions and simulation questions to understand the subtlety of the questions.