In addition and subtraction within 40, we learn the following:
- addition without regrouping
- addition with regrouping
- subtraction without regrouping
- subtraction with regrouping
- word problems
Addition without regrouping
When adding numbers within 40, we can use either one way:
1. Make use of number bonds
2. Using the addition work method
Let's use the example of 23 + 12 to illustrate both methods:
Method 1:
Split 23 = 20 +3
Split 12 = 10 +2
10 + 20 = 30
2 + 3 = 5
30 + 5 =35
Hence, 23 +12 = 35
Method 2:
We'll write the addition statement vertically, with aligning numbers in the ones place together; and numbers in the tens place together.
2 3
+ 1 2
____
3 5
____
This method is the more commonly used method as the numbers get bigger.
Addition with regrouping
In addition with regrouping, when the numbers in the ones place are added together, we end up having a number greater than 10. Since a number greater than 10 consists of a ones- place and a tens- place, we'll need to add a number to the tens place.
Subtraction without regrouping
When subtracting numbers, without regrouping, we could also write the numbers vertically. Next, subtract the numbers in the ones place with the number in the ones place; then subtract the number in the tens place with the number in the tens place.
32
- 11
____
21
____
Subtraction with regrouping
We subtract numbers with regrouping, we'll add ten to the ones place, and subtract ten.
For instance, let's look at 32 - 17:
32
- 17
____
15
____
Word Problems
Students apply the addition and subtraction that they have learnt to solve word problems.
