Subtracting round numbers

### 1.1. Knowledge to remember

– Know how to calculate, calculate and subtract round numbers

– Mentally subtract whole numbers within 90

– Solve problems with subtraction.

### 1.2. Math forms on Subtracting round numbers

**Form 1: Performing the calculation**

Perform the subtraction of two decimals by subtracting the numbers in the ones place and then subtracting 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 subtractions of rounded numbers, you need to perform the calculation and then compare the results you have just found.

### 1.3. Solving Textbook Exercises page 131

**Lesson 1 page 131**

Calculate:

**Solution method**

Subtract numbers from right to left.

__Solution guide__

**Lesson 2 page 131**

Mental arithmetic:

50 – 30 = ?

Mental: 5 tens – 3 tens = 2 tens

So: 50 – 30 = 20.

40 – 30 = 80 – 40 =

70 – 20 = 90 – 60 =

90 – 10 = 50 – 50 =

__Solution method__

Subtract the tens place and then add a 0 to the end of the result you just found.

__Solution guide__

40 – 30 =

Mental 4 tens – 3 tens = 1 dozen

So: 40 – 30 = 10.

Similarly, I enter the result as follows:

40 – 30 = 10 80 – 40 = 40

70 – 20 = 50 90 – 60 = 30

90 – 10 = 80 50 – 50 = 0.

**Lesson 3 page 131**

An has 30 candies, she gives him 10 more. How many candies does An have in all?

__Solution method__

**Summary**

Yes : 30 candies

Extra: 10 candies

All: … the candy?

**The answer**

To find the solution, we take the number of candies An has plus the number of candies An is given.

__Solution guide__

An’s total number of candies is:

30 + 10 = 40 (candy)

Answer: 40 candies.

**Lesson 4 page 131**

Put the appropriate <, >, = in the dot:

50 – 10 …20 40 – 10….40 30….50 – 20

__Solution method__

– Find the value on each side

– Compare tens digits, the number with larger tens is larger.

**Solution guide**

50 – 10 > 20 40 – 10 < 40 30 = 50 - 20

### 1.4. Solving Textbook Exercises page 132

**Lesson 1 page 132**

Calculate then calculate:

70 – 50 60 – 30 90 – 50

80 – 40 40 – 10 90 – 40

**Solution method**

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

– Calculation: Perform subtraction from right to left.

__Solution guide__

**Lesson 2 page 132**

Number?

__Solution method__

Calculate from left to right and then fill in the blank with the result.

__Solution guide__

Do the following calculation:

90 – 20 = 70 70 – 30 = 40 40 – 20 = 20 20 + 10 = 30

Fill in the blanks, we get the following result:

**Lesson 3 page 132**

Correct write d, false write s:

__Solution method__

– Subtract the numbers and then write the unit after the result you just found.

– Check the given calculations and then fill in the blanks with dd or s.

__Solution guide__

a) S (missing cm)

B d

c) S (wrong result of subtraction).

**Lesson 4 page 132**

Lan’s family has 20 bowls, mother buys another dozen bowls. How many bowls does Lan have in all?

__Solution method__

**Summary**

Yes: 20 bowls

Extra: 1 dozen bowls

All : … the bowl?

**The answer**

To find the solution, we take the original number of bowls plus the number of additional bowls purchased.

__Solution guide__

1 dozen bowls = 10 bowls

The number of bowls in Lan’s house is:

20 + 10 = 30 (bowl)

Answer: 30 bowls.

**Lesson 5 page 132**

Put the appropriate + or – in the dot to get the calculation correct.

50…10 = 40 30…20 = 50 40…20 = 20

__Solution method__

Observe the result of the calculation increasing or decreasing from the original number and then use the appropriate addition or subtraction.

**Solution guide**

50 – 10 = 40 30 + 20 = 50 40 – 20 = 20

.

=====

## Leave a Reply