Subtraction within 100 (minus not remembered)

### 1.1. Knowledge to remember

– Know how to calculate and subtract two-digit numbers.

– Mentally subtract numbers within 100;

– Apply to solve word problems.

### 1.2. Mathematical forms on Subtraction within 100 (minus minus)

**Form 1: Performing the calculation**

Perform the subtraction of two-digit numbers by working from right to left, subtracting the units numbers, and then subtracting the tens numbers.

**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.

Often the problem requires finding the “remaining” value, we will use subtraction to find the solution.

– 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 values of the subtraction of two-digit numbers, you need:

– Do the math

– Compare the results just found.

### 1.3. Solving Textbook Exercises page 158

**Lesson 1 page 158**

a) Calculate

b) Calculate then calculate

67 – 22 56 – 16 94 – 92 42 – 42 99 – 66

**Solution method**

a) Subtract numbers from right to left.

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

– Calculation: Subtract from right to left in turn.

__Solution guide__

**Lesson 2 page 158**

Correct write d, false write s:

__Solution method__

Check the calculation and calculation of the subtraction and then fill in the blanks with dd or s.

__Solution guide__

Do the math:

a) 87 – 35 = 52 68 – 21 = 47 95 – 24 = 71 43 – 12 = 31

b) 57 – 23 = 34 74 – 11 = 63 88 – 80 = 8 47 – 47 = 0

Compare the calculated results with the given results, fill in the blanks, we get the following results:

**Lesson 3 page 158**

Lan’s book consists of 64 pages, Lan has read 24 pages. How many more pages does Lan have to read before the book is over?

__Solution method__

**Summary**

All: 64 pages

Read: 24 pages

Left: … pages?

**The answer**

To find the solution, we subtract 64 pages from the number of pages read.

**Solution guide**

The number of pages that Lan still has to read to finish the book is:

64 – 24 = 40 (pages)

Answer: 40 pages.

### 1.4. Textbook Exercises Page 159

**Lesson 1 page 159**

Calculate

**Solution method**

Subtract numbers from right to left.

__Solution guide__

**Lesson 2 page 159**

Correct write d, false write s:

__Solution method__

Check the calculation and calculation of the given operations and then fill in dd or s in the blanks.

– Set calculation: Numbers in the same row need to be written in a straight column.

– Calculation: Need to subtract from right to left in turn.

__Solution guide__

**Lesson 3 page 159**

Mental arithmetic:

a) 66 – 60 = 98 – 90 = 72 – 70 =

78 – 50 = 59 – 30 = 43 – 20 =

b) 58 – 4 = 67 – 7 = 99 – 1 =

58 – 8 = 67 – 5 = 99 – 9 =

__Solution method__

Subtract the numbers and then fill in the blanks with the results.

__Solution guide__

a) 66 – 60 = 6 98 – 90 = 8 72 – 70 = 2

78 – 50 = 28 59 – 30 = 29 43 – 20 = 23

b) 58 – 4 = 54 67 – 7 = 60 99 – 1 = 98

58 – 8 = 50 67 – 5 = 62 99 – 9 = 90

### 1.5. Solving Textbook Exercises page 160

**Lesson 1 page 160**

Calculate then calculate:

45 – 23 57 – 31 72 – 60 70 – 40 66 – 25

**Solution method**

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

– Calculation: Subtract numbers from right to left in turn.

__Solution guide__

**Lesson 2 page 160**

Mental arithmetic:

65 – 5 = 65 – 60 = 65 – 65 =

70 – 30 = 94 – 3 = 33 – 30 =

21 – 1 = 21 – 20 = 32 – 10 =

__Solution method__

Perform mental subtraction of numbers and then fill in the blank with the result.

__Solution guide__

65 – 5 = 60 65 – 60 = 5 65 – 65 = 0

70 – 30 = 40 94 – 3 = 91 33 – 30 = 3

21 – 1 = 20 21 – 20 = 1 32 – 10 = 22

**Lesson 3 page 160**

Enter <; >; = fits in the dot:

35 – 5 …. 35 – 4 43 + 3 ….. 43 – 3

30 – 20 …. 40 – 30 31 + 42 ….. 41 + 32

__Solution method__

– Calculate the value of the sides.

Compare and then fill in the blanks with the appropriate mark.

__Solution guide__

35 – 5 < 35 - 4 43 + 3 > 43 – 3

30 – 20 = 40 – 30 31 + 42 = 41 + 32

**Lesson 4 page 160**

Class 1B has 35 students, including 20 girls. How many boys are there in class 1B?

__Solution method__

**Summary **

All : 35 friends

Female: 20 friends

Nam: … you?

To find the solution, we subtract the number of students in class 1B from the number of girls we know.

__Solution guide__

Class 1B has the number of boys:

35 – 20 = 15 (you)

Answer: 15 friends.

**Lesson 5 page 160**

Join (according to the pattern)

__Solution method__

– Calculate the value of the given operations.

– Match with the cell containing the appropriate result.

**Solution guide**

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