Math 2 Lesson: One-fifth

### 1.1. Knowledge to remember

Understand the concept of \(\frac{1}{5}\), recognize the image and spelling of “one-fifth”.

### 1.2. Math forms

**Type 1: Check if a shape has been colored \(\frac{1}{5}\) shape or not.**

– The given figure is divided into 5 equal parts.

– Color 1 out of 5 parts.

If the picture meets the above two conditions, the figure has been colored \(\frac{1}{5}\) the figure.

**Form 2: Find \(\frac{1}{5}\) given number of squares.**

– Count the total number of squares

– Take the total number of squares divided by 5, the result of the division is \(\frac{1}{5}\) the number of squares.

**Form 3: Practice the 5 bảng division table**

Repeat the 5 division table you have learned to find the value of the previous division chia

### 1.3. Solving Textbook Exercises page 122

**Lesson 1**

Which shape has been colored \(\displaystyle{1 \over 5}\)?

__Solution method__

Choose the shape that has \(5\) equal parts and has a colored part.

__Solution guide__

Colored \(\displaystyle{1 \over 5}\) of image A, figure D.

**Lesson 2**

Which shape has colored \(\displaystyle{1 \over 5}\) the number of squares?

__Solution method__

– Count the number of squares in each shape and find \(\dfrac{1}{5}\) the number of squares.

– Count the number of filled squares, if equal to the calculated value, the figure has \(\dfrac{1}{5}\) the number of colored squares.

__Solution guide__

**Picture A:** There are \(10\) squares.

We have: \(10:5=2\) and figure A has exactly \(2\) colored squares,

**Figure B: **There are \(15\) squares.

We have: \(15:5=3\) but picture B has \(5\) colored squares.

**Figure C: **There are \(15\) squares.

We have: \(15:5=3\) and figure C has exactly \(3\) colored squares.

**Figure D: **There are \(20\) squares.

We have: \(20:5=4\) which the picture has colored \(5\) squares.

So image A and figure C are colored shapes \(\displaystyle{1 \over 5}\) number of squares.

**lesson 3**

Which figure circled \(\displaystyle{1 \over 5}\) the number of ducks?

__Solution method__

– Count the number of ducks in each picture and then find \(\dfrac{1}{5}\) of that number.

– Choose the answer that is circled with the correct number of ducks equal to the number you just found.

__Solution guide__

Two pictures with \(10\) duck.

We have: \(10 : 5=2\) duck.

So figure a has circled \(\displaystyle{1 \over 5}\) the number of ducks.

### 1.4. Solve the textbook exercises page 123

**Lesson 1**

Mental arithmetic:

10 : 5 = 15 : 5 = 20 : 5 = 25 : 5 =

30 : 5 = 45 : 5 = 35 : 5 = 50 : 5 =

__Solution method__

Review the division table by 5 and then fill in the space with the result of the division.

__Solution guide__

10 : 5 = 2 15 : 5 = 3 20 : 5 = 4 25 : 5 = 5

30 : 5 = 6 45 : 5 = 9 35 : 5 = 7 50 : 5 = 10

**Lesson 2**

Mental arithmetic:

5 x 2 = 5 x 3 = 5 x 4 = 5 x 1 =

10 : 2 = 15 : 3 = 20 : 4 = 5 : 1 =

10 : 5 = 15 : 5 = 20 : 5 = 5 : 5 =

__Solution method__

Find the value of the multiplication and then quickly fill in the results of the 2 related divisions.

__Solution guide__

5 x 2 = 10 5 x 3 = 15 5 x 4 = 20 5 x 1 = 5

10 : 2 = 5 15 : 3 = 5 20 : 4 = 5 5 : 1 = 5

10 : 5 = 2 15 : 5 = 3 20 : 5 = 4 5 : 5 = 1

**lesson 3**

There are 35 books divided equally among 5 friends. How many books do each of you have?

__Solution method__

To find the solution, we divide the number of books by 5.

__Solution guide__

The number of notebooks for each student is:

35 : 5 = 7 (book)

Answer: 7 notebooks.

**Lesson 4**

There are 25 oranges arranged on the plates, each plate has 5 oranges. How many discs can it be arranged?

__Solution method__

To find the solution, divide the number of oranges by 5.

__Solution guide__

The number of cam discs is:

25 : 5 = 5 (disc)

Answer: 5 orange discs.

**Lesson 5**

Which figure circled \(\dfrac{1} {5}\) the number of elephants?

__Solution method__

– Count the number of elephants and divide that number by 5.

– Choose the picture with the number of elephants circled by the result of the division just found.

__Solution guide__

Each picture has 15 elephants.

We have: 15 : 5 = 3

So figure a has circled \(\dfrac{1} {5}\) the number of elephants.

### 1.5. Solving Textbook Exercises page 124

**Lesson 1**

Calculation (according to the form):

**Sample: 3 x 4 : 2 = 12 : 2 = 6**

a) 5 x 6 : 3 =

b) 6 : 3 x 5 =

c) 2 x 2 x 2 =

__Solution method__

Calculate in turn from left to right according to the sample.

__Solution guide__

a) 5 x 6 : 3 = 30 :3 = 10

b) 6 : 3 x 5 = 2 x 5 = 10

c) 2 x 2 x 2 = 4 x 2 = 8

**Lesson 2**

Find x:

a) x + 2 = 6

x × 2 = 6

b) 3 + x = 15

3 × x = 15

__Solution method__

– To find the term, we subtract the other term from the sum.

To find a factor, we divide the product by the other factor.

__Solution guide__

a) x + 2 = 6

x = 6 − 2

x = 4

x × 2 = 6

x = 6 : 2

x = 3

b) 3 + x = 15

x = 15 − 3

x=12

3 × x = 15

x = 15 : 3

x = 5

**lesson 3**

Which image has been colored:

\({1 \over 2}\) number of squares?

\({1 \over 3}\) number of squares?

\({1 \over 4}\) number of squares?

\({1 \over 5}\) number of squares?

__Solution method__

– Applying knowledge learned about a second, a third … learned.

– The image is divided into \(n\) equal parts and has \(1\) the painted part, then we say the painted part is \(\dfrac{1}{n}\) the original shape.

__Solution guide__

Figure C has been colored \({1 \over 2}\) number of squares.

Figure A has been colored \({1 \over 3}\) the number of squares.

Figure D has been colored with \({1 \over 4}\) the number of squares.

Figure B has been colored with \({1 \over 5}\) the number of squares.

**Lesson 4**

Each cage has 5 rabbits. How many rabbits are there in 4 such cages?

__Solution method__

To find the solution, we multiply 5 times the number of cages.

__Solution guide__

The total number of rabbits is:

5 x 4 = 20 (rabbit)

Answer: 20 rabbits.

**Lesson 5**

Arrange 4 triangles into a rectangle:

__Solution method__

Match the given triangles into a rectangle.

__Solution guide__

We have the following arrangement:

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