12 minus a number: 12-8

### 1.1. Knowledge to remember

### 1.2. Math forms

**Form 1: Calculate**

– Set alignment.

– Perform subtraction in turn from right to left, when the unit row of the number being subtracted is not enough to subtract, you need to perform the subtraction with memory.

**Form 2: Build the math and find the result.**

– Write calculations according to the data of the given problem.

– Perform calculations to find the value of the result.

**Form 3: Math Quiz**

– Read and analyze the topic

– Find the solution of the problem: Pay attention to the keywords “all”; “remaining”… to use appropriate math in solving problems.

– Presentation of the problem.

– Check the solution and the results you just found.

### 1.3. Solving Textbook Exercises

**Lesson 1**

Mental arithmetic:

a) 9 + 3 = 8 + 4 = 7 + 5 = 6 + 6 =

3 + 9 = 4 + 8 = 5 + 7 = 12 – 6 =

12 – 9 = 12 – 8 = 12 – 7 =

12 – 3 = 12 – 4 = 12 – 5 =

b) 12 – 2 – 7 = 12 – 2 – 5 = 12 – 2 – 6 =

12 – 9 = 12 – 7 = 12 – 8 =

__Solution method__

Mentally find the results of calculations and then fill in the blanks.

__Solution guide__

a) 9 + 3 = 11 8 + 4 = 12 7 + 5 = 12 6 + 6 = 12

3 + 9 = 11 4 + 8 = 12 5 + 7 = 12 12 – 6 = 6

12 – 9 = 3 12 – 8 = 4 12 – 7 = 5

12 – 3 = 9 12 – 4 = 8 12 – 5 = 7

b) 12 – 2 – 7 = 3 12 – 2 – 5 = 5 12 – 2 – 6 = 4

12 – 9 = 3 12 – 7 = 5 12 – 8 = 4

**Lesson 2**

Calculate:

__Solution method__

Perform the subtraction and then fill in the blank with the result.

__Solution guide__

**lesson 3**

Set the calculation and then calculate the difference, knowing the subtracted and subtracted numbers are:

a) 12 and 7; b) 12 and 3; c) 12 and 9.

__Solution method__

– Set calculation: Write the calculation so that the digits are in the same row and column together.

– Calculation: Perform subtraction and write the result in the blank.

__Solution guide__

**Lesson 4**

There are 12 notebooks, of which 6 are red-covered, the rest are blue-bound. How many green cover books are there?

__Solution method__

To find the blue-covered notebook, subtract the number of the original notebook from the number of red-covered notebooks.

__Solution guide__

The number of green cover books is:

12 – 6 = 6 (book)

Answer: 6 books.

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