Number

Addition and Subtraction within 20

Junior Infants · Senior Infants · 1st Class · 2nd Class

✓By the end of this lesson students will be able to understand addition as 'putting together' and subtraction as 'taking away'.
✓By the end of this lesson students will be able to recall number bonds to 10 and 20.
✓By the end of this lesson students will be able to use mental strategies like doubles and near-doubles for addition.
✓By the end of this lesson students will be able to solve addition and subtraction problems within 20.

Key concepts

Addition

Addition means putting groups of things together to find a total. We use the '+' symbol for addition. For example, 3 + 2 means 3 and 2 more, making 5.

Subtraction

Subtraction means taking away a part from a whole group to find what is left. We use the '-' symbol for subtraction. For example, 5 - 2 means 5 take away 2, leaving 3.

Number Bonds

Number bonds are pairs of numbers that add up to a given total. Knowing your number bonds helps you add and subtract quickly without counting every time. For example, the number bonds for 10 are 1+9, 2+8, 3+7, 4+6, 5+5, and 0+10.

Doubles

Doubles are when you add a number to itself. For example, the double of 3 is 3 + 3 = 6. Knowing your doubles helps you add faster.

Near-Doubles

Near-doubles are when you add two numbers that are very close to each other, like 4 + 5. You can think of it as a double plus one (4 + 4 + 1 = 9) or a double minus one (5 + 5 - 1 = 9). This is a helpful mental strategy.

Key facts to remember

1Addition means putting groups together to find a total.
2Subtraction means taking away a part from a group to find what is left.
3Number bonds are pairs of numbers that add up to a specific total (like 10 or 20).
4Knowing your number bonds helps you add and subtract quickly.
5Doubles are when you add a number to itself (e.g., 4 + 4 = 8).
6Near-doubles are numbers that are close to each other (e.g., 4 + 5 can be solved using 4 + 4 + 1).
7You can use your fingers or objects to help you count.
8Always check your answer by counting again.

Worked examples

Example 1

What is 7 + 3?

IWe need to find the total when 7 and 3 are put together.

IIThink of your number bonds to 10. What number goes with 7 to make 10?

III7 and 3 make 10.

Answer

7 + 3 = 10

Knowing your number bonds to 10 makes this very quick!

Example 2

There are 10 apples. You eat 4 apples. How many apples are left?

IWe start with 10 apples and take away 4.

IIThis is a subtraction problem: 10 - 4.

IIIThink of your number bonds to 10. What number goes with 4 to make 10?

IV4 and 6 make 10, so 10 - 4 = 6.

Answer

6 apples are left.

Subtraction is the opposite of addition. If 4 + 6 = 10, then 10 - 4 = 6.

Example 3

Add 6 + 7.

IWe need to find the total of 6 and 7.

IIThink of the double of 6: 6 + 6 = 12.

IIISince 7 is one more than 6, 6 + 7 is one more than 6 + 6.

IVSo, 12 + 1 = 13.

Answer

6 + 7 = 13

You could also think of the double of 7: 7 + 7 = 14. Then 6 is one less than 7, so 6 + 7 is one less than 7 + 7: 14 - 1 = 13.

Common mistakes

✗Confusing the '+' (add) and '-' (subtract) symbols.
✗Incorrectly recalling number bonds, especially for numbers over 10.
✗Making counting errors when using fingers or objects (e.g., skipping a number or counting one twice).
✗Not understanding that 'take away' means subtraction and 'altogether' means addition.

Exam tips

★Read the question carefully to know if you need to add or subtract.
★Practise your number bonds to 10 and 20 every day until you know them by heart.
★If you are stuck, use your fingers, draw a picture, or use small objects to help you count.
★Always double-check your answer by counting forwards or backwards again.

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