Review addition and subtraction

### 1.1. Knowledge to remember

– How to find the value of addition and subtraction within 1000.

– How to set and calculate the vertical column of addition and subtraction of three-digit numbers.

### 1.2. Types of Maths

**Form 1: Set and calculate**

– Set the calculation with the corresponding rows to line the columns together: Hundreds of one number line hundreds of another, and so on with tens and units.

– Perform subtraction from right to left.

**Form 2: Mental calculation of addition and subtraction with round numbers**

– Add hundreds digits. Add two zeros to the end of the result you just found.

– Perform mental subtraction of round numbers by subtracting hundreds and keeping tens and units.

**Form 3: Math Quiz**

– Read and analyze problems: Problems for the value of quantities or problems about “more” or “less than”.

– Find a solution: To find “all” or the value of a quantity more or less, we often perform addition or subtraction of numbers.

– Presenting the solution

– Check the results and the solution of the problem.

### 1.3. Solving Textbook Exercises page 170

**Lesson 1**

Mental arithmetic:

30 + 50 = 70 – 50 = 300 + 200 =

20 + 40 = 40 + 40 = 600 – 400 =

90 – 30 = 60 – 10 = 500 + 300 =

80 – 70 = 50 + 40 = 700 – 400 =

__Solution method__

Add and subtract non-zero numbers and then add one or two zeros to the end of the result you just found.

__Solution guide__

30 + 50 = 80 70 – 50 = 20 300 + 200 = 500

20 + 40 = 60 40 + 40 = 80 600 – 400 = 200

90 – 30 = 60 60 – 10 = 50 500 + 300 = 800

80 – 70 = 10 50 + 40 = 90 700 – 400 = 300

**Lesson 2**

Calculate:

__Solution method__

Perform addition or subtraction in order from right to left.

__Solution guide__

**lesson 3**

An elementary school has 265 girls and 234 boys. How many students does that elementary school have?

__Solution method__

**Summary**

Girls: 265 students

Boys: 234 students

All: ? the student.

**The answer**

To find the solution, we take the number of girls plus the number of boys.

__Solution guide__

That elementary school has a student population of:

265 + 234 = 499 (student)

Answer: 499 students.

**Lesson 4**

The first tank holds \(865\ell\) water, the second tank holds less than the first tank \(200\ell\) water. How many liters of water can the second tank hold?

__Solution method__

*Summary*

To find the solution we take the number of liters of water in the first tank minus \(200\ell\).

__Solution guide__

The number of liters of water contained in the second tank is:

\(865 – 200 = 665 (\ell)\)

Answer: \(665\ell\).

### 1.4. Solving Textbook Exercises page 171

**Lesson 1**

Mental arithmetic:

500 + 300 = 400 + 300 = 700 + 100 =

800 – 500 = 600 – 400 = 800 – 700 =

800 – 300 = 600 – 200 = 800 -100 =

__Solution method__

Add and subtract hundreds, and then add two zeros to the end of the result.

__Solution guide__

500 + 300 = 800 400 + 300 = 700 700 + 100 = 800

800 – 500 = 300 600 – 400 = 200 800 – 700 = 100

800 – 300 = 500 600 – 200 = 400 800 -100 = 700

**Lesson 2**

Calculate then calculate:

a) 65 + 29 55 + 45 100 – 72

b) 345 + 422 674 – 353 517 + 360

__Solution method__

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

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

__Solution guide__

**lesson 3**

I am 165cm tall, you are 33cm shorter than me. How many centimeters tall are you?

__Solution method__

**Summary**

English: 165cm

I’m shorter than you: 33cm

Me: …cm?

**The answer**

To find the solution, we subtract 33cm from his height.

__Solution guide__

My height is:

165 – 33 = 132 (cm)

Answer: 132cm.

**Lesson 4**

Team One planted 530 trees, Team Two planted 140 trees more than Team One. How many trees can Team Two plant?

__Solution method__

**Summary**

Team One: 530 plants

Team Two is more than Team One: 140 plants

Team Two: ….tree?

**The answer**

To find the answer, we take the number of trees that Team One can plant plus 140 trees.

__Solution guide__

The number of plants that Team Two can plant is:

530 + 140 = 670 (tree)

Answer: 670 trees.

Find x:

a) x – 32 = 45

b) x + 45 = 79

__Solution method__

– To find the number to be subtracted we take the difference of addition and subtraction.

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

__Solution guide__

a) x – 32 = 45

x = 45 + 32

x = 77

b) x + 45 = 79

x = 79 – 45

x = 34

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