Add the numbers round tens

### 1.1. Knowledge to remember

– Know how to calculate, do the addition of ten round numbers.

– Mentally add numbers in the tens of 90;

– Solve problems with addition.

– The first step to know about the property of addition: When swapping two numbers in an addition, the result does not change.

Calculation: 30 + 20

0 plus 0 equals 0, write 0.

3 plus 2 equals 5, write 5.

So 30 + 20 = 50.

### 1.2. Math forms on Addition of round numbers tròn

Form 1: Performing the calculation

Perform the addition of two decimals by adding the numbers in the ones place and then adding the numbers in the tens place.

Form 2: Math problems with words

– Read and analyze the problem: Determine the given numbers, the number of increases or decreases and the requirements of the problem.

– Find the solution to the problem: I rely on the keywords “add”, “less”, “all”, “remaining”… to determine the calculation needed for the problem.

– Present the solution to the problem: Write clearly the solution, calculation, answer.

– Check the solution and the results you just found.

Form 3: Compare

To compare two or more additions of rounded numbers:

- Step 1: Do the math.
- Step 2: Compare the results just found.

### 1.3. Solving Textbook Exercises page 129

Lesson 1 page 129

Calculate:

__Solution method__

Add the numbers from right to left.

__Solution guide__

Lesson 2 page 129

Mental arithmetic:

20 + 30 = ?

Mental 2 tens + 3 tens = 5 tens

So 20 + 30 = 50

50 + 10 = 40 + 30 = 50 + 40 =

20 + 20 = 20 + 60 = 40 + 50 =

30 + 50 = 70 + 20 = 20 + 70 =

__Solution method__

Mentally calculate according to the form and fill in the blank with the result.

__Solution guide__

50 + 10 =

Mental: 5 tens + 1 dozen = 6 tens.

So 50 + 10 = 60.

Doing the same mental calculation, I enter the results as follows:

50 + 10 = 60 40 + 30 = 70 50 + 40 = 90

20 + 20 = 40 20 + 60 = 80 40 + 50 = 90

30 + 50 = 80 70 + 20 = 90 20 + 70 = 90

Lesson 3 page 129

The first box contains 20 packages of cakes, the second box contains 30 packages of cakes. How many cakes are in both boxes?

__Solution method__

*Summary*

The first box: 20 packs of cakes

Second box: 30 packs of cakes

Both: … cake package?

*The answer*

To find the solution, we take the number of cakes in the first box plus the number of cakes in the second box.

__Solution guide__

Both boxes contain the number of packages of cakes:

20 + 30 = 50 (cake pack)

Answer: 50 packages of cakes.

### 1.4. Solving Textbook Exercises page 130

Lesson 1 page 130

Calculate then calculate:

40 + 20 10 + 70 60 + 20

30 + 30 50 + 40 30 + 40

__Solution method__

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

– Calculation: Add the numbers from right to left in turn.

__Solution guide__

Lesson 2 page 130

Mental arithmetic:

a) 30 + 20 = 40 + 50 = 10 + 60 =

20 + 30 = 50 + 40 = 60 + 10 =

b) 30cm + 10cm = 50cm + 20cm =

40cm + 40cm = 20cm + 30cm =

__Solution method__

a) Add the decimals and write the result in the blanks.

b) Add the numbers and then add the units of centimeters after the results found.

__Solution guide__

a) 30 + 20 = 50 40 + 50 = 90 10 + 60 = 70

20 + 30 = 50 50 + 40 = 90 60 + 10 = 70

b) 30cm + 10cm = 40cm 50cm + 20cm = 70cm

40cm + 40cm = 80cm 20cm + 30cm = 50cm

Lesson 3 page 130

Lan picked 20 flowers, Mai picked 10 flowers. How many flowers did you both pick?

__Solution method__

*Summary*

Orchid: 20 flowers

Plum: 10 flowers 10

Both: … the flower?

*The answer*

Want to find the solution, we take the number of flowers that Lan picked plus the number of flowers that Mai picked.

__Solution guide__

The number of flowers you both picked is:

20 + 10 = 30 (flower)

Answer: 30 flowers.

Lesson 4 page 130

Join (according to the pattern)

__Solution method__

– Perform the addition of the given ten numbers.

– Match with the appropriate result.

__Solution guide__

Perform the following permission:

60 + 20 = 80

40 + 40 = 80

40 + 30 = 70

20 + 20 = 40

10 + 40 = 50

10 + 60 = 70

30 + 10 = 40

30 + 20 = 50

Combining the calculations just calculated, we get the following result:

.

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