The subtraction of 30 three-digit numbers, a quasi-junior three student, will be completed in six minutes. It takes about six minutes to solve 30 math problems in the third grade, but some slightly more difficult ones may take ten minutes, like my children.

It may take five or six minutes if the basic 30 questions are simple, but it may take a little longer if it is a mixed calculation of multiplication, division, addition and subtraction.

Design concept

In teaching, students can continue to make up stories on the basis of information window 2 and find out the information provided in the situation map. Then, according to the questions raised in the situation diagram, they can list formulas and enter the study of new knowledge.

Guide students to estimate first, and then calculate the accurate result vertically. Vertical computing allows students to try independently and then communicate in groups. The key to communication is to guide students to make it clear that "ten 9 plus 8 equals 17". How to write? Why? " Make students understand the algorithm of "add up decimal 1" and master the algorithm.

If students have difficulty in understanding, they can use counters to help them understand the breakthrough points. After the teaching, teachers should guide students to discuss "what should be paid attention to when adding and subtracting vertically", and combine examples to sort out and summarize the vertical calculation methods of three-digit addition and subtraction.