Review multiplication and division

### 1.1. Knowledge to remember

**a) Multiplication**

Understand the relationship between the sum of the same terms with multiplication.

– Know how to write, read and calculate the result of multiplication.

2 + 2 + 2 + 2 + 2 = 10

2 + 2 + 2 + 2 + 2 is the sum of 5 equal terms, each of which is 2.

We convert to multiplication, write as follows: 2 x 5 = 10

Read as *Two times five equals ten.*

Sign **x** called the multiplication sign.

**b) Division**

The relationship between division and multiplication.

– Read, write and calculate the result of division.

### 1.2. Math forms

**Form 1: Convert the sum of equal terms into multiplication and v**Calculate division from a given multiplication.

– Determine the value of each term.

Determine the number of equal terms in a given sum.

– Multiply the two numbers you just found.

– From the given multiplication, you divide the product by one factor to get the value equal to the other factor.

**Form 2: Calculate the value of multiplication and division**

– I mentally follow the addition of the same numbers to find the value of the desired multiplication.

**Form 3: Math Quiz**

– Read and analyze the problem: The problem often gives the same number of values, requires finding the value of some groups.

– Find the solution of the problem: To find the value of some groups, we usually take the value of a term multiplied by the number of groups.

– Presentation of the problem.

– Check the solution and the results you just found.

### 1.3. Solving Textbook Exercises page 172

**Lesson 1**

Mental arithmetic:

a)

2 x 8 = 12 : 2 = 2 x 9 = 18 : 3 =

3 x 9 = 12 : 3 = 5 x 7 = 45 : 5 =

4 x 5 = 12 : 4 = 5 x 8 = 40 : 4 =

5 x 6 = 15 : 5 = 3 x 6 = 20 : 2 =

b)

20 x 4 = 30 x 3 = 20 x 2 = 30 x 2 =

80 : 4 = 90 : 3 = 40 : 2 = 60 : 2 =

__Solution method__

a) Mentally calculate the value of multiplication and division in the learned range.

b) Calculate with tens digits and then add a 0 to the end of the result.

__Solution guide__

a)

2 x 8 = 16 12 : 2 = 6 2 x 9 = 18 18 : 3 = 6

3 x 9 = 27 12 : 3 = 4 5 x 7 = 35 45 : 5 = 9

4 x 5 = 20 12 : 4 = 3 5 x 8 = 40 40 : 4 = 10

5 x 6 = 30 15 : 5 = 3 3 x 6 = 18 20 : 2 = 10

b)

20 x 4 = 80 30 x 3 = 90 20 x 2 = 40 30 x 2 = 60

80 : 4 = 20 90 : 3 = 30 40 : 2 = 20 60 : 2 = 30

Lesson 2

Calculate:

4 x 6 + 16 =

20 : 4 x 6 =

5 x 7 + 25 =

30 : 5 : 2 =

__Solution method__

Perform calculations in order from left to right.

__Solution guide__

4 x 6 + 16 = 24 + 16 = 40

20 : 4 x 6 = 5 x 6 = 30

5 x 7 + 25 = 35 + 25 = 60

30 : 5 : 2 = 6 : 2 = 3

Student 2A is arranged in 8 rows, each row has 3 students. How many students are there in class 2A?

__Solution method__

**Summary**

1 row: 3 students

8 rows: ? the student

**The answer**

To find the answer, we multiply the number of students in a row by 8.

__Solution guide__

The number of students in class 2A is:

3 x 8 = 24 (students)

Answer: 24 students.

Which figure circled the \(\displaystyle {1 \over 3}\) number of circles?

__Solution method__

Count the number of circles and divide by 3.

– Select the picture with the number of circles circled by the result of the division calculation just found.

__Solution guide__

Each shape has 15 circles.

Which 15 : 3 = 5

So figure a) is the circled shape \(\displaystyle {1 \over 3}\) the number of circles.

Find x:

a) x : 3 = 5

b) 5 xx = 35.

__Solution method__

– To find the divided number, we take the merchant with the divisor.

To find a factor, divide the product by the known factor.

__Solution guide__

a) x : 3 = 5

x = 5 x 3

x= 15

b) 5 xx= 35

x = 35 : 5

x = 7

### 1.4. Textbook Exercises Page 173

Lesson 1

Mental arithmetic:

4 x 9 = 5 x 7 = 3 x 8 = 2 x 8 =

36 : 4 = 35 : 5 = 24 : 3 = 16 : 2 =

__Solution method__

Mentally calculate the value of the multiplication and then quickly fill in the result of the division involved.

__Solution guide__

4 x 9 = 36 5 x 7 = 35 3 x 8 = 24 2 x 8 = 16

36 : 4 = 9 35 : 5 = 7 24 : 3 = 8 16 : 2 = 8

Lesson 2

Calculate:

2 x 2 x 3 = 3 x 5 – 6 =

40 : 4 : 5 = 2 x 7 + 58 =

4 x 9 + 6 = 2 x 8 + 72 =

__Solution method__

Perform calculations in order from left to right.

__Solution guide__

2 x 2 x 3 = 4 x 3 = 12 3 x 5 – 6 = 15 – 6 = 9

40 : 4 : 5 = 10 : 5 = 2 2 x 7 + 58 = 14 + 58 = 72

4 x 9 + 6 = 36 + 6 = 42 2 x 8 + 72 = 16 + 72 = 88

lesson 3

There are 27 pencils divided equally among 3 groups. How many crayons are there in each group?

__Solution method__

*Summary*

3 groups: 27 pencils

1 group: … pencils?

**The answer**

To find the solution we divide 27 by the number of groups.

__Solution guide__

The number of crayons in each group is:

27 : 3 = 9 (pencil)

Answer: 9 pencils.

Which figure circled \({1 \over 4}\) the number of squares?

__Solution method__

Count the number of squares and divide by 4.

– Select the picture with the number of squares circled by the result of the division just calculated.

__Solution guide__

Figure a has 20 squares and 20 : 4 = 5 but figure a has 4 circled squares.

Figure b has 16 squares and 16 : 4 = 4.

Figure b has exactly 4 circled squares.

So figure b is the figure with \({1 \over 4}\) the number of squares circled.

Number ?

4 + … = 4 … x 4 = 0

4 – … = 4 … : 4 = 0

__Solution method__

Fill in the blanks with the appropriate number, paying attention to the knowledge of the number 1; 0 in multiplication and division.

__Solution guide__

4 + 0 = 4 0 x 4 = 0

4 – 0 = 4 0 : 4 = 0

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