Number & Algebra
Addition and Subtraction within 100: Building Number Facts
Year 1 · Year 2
- ✓By the end of this lesson students will be able to recall addition and subtraction number facts for sums to 20.
- ✓By the end of this lesson students will be able to use various strategies (e.g., counting on, partitioning) to solve addition problems within 100.
- ✓By the end of this lesson students will be able to use various strategies (e.g., counting back, partitioning) to solve subtraction problems within 100.
- ✓By the end of this lesson students will be able to understand the inverse relationship between addition and subtraction.
Key concepts
Number facts are basic addition and subtraction sums that you know 'in your head' very quickly. For example, knowing that 3 + 2 = 5 or 10 - 4 = 6 without needing to count. Knowing these facts helps you solve bigger maths problems much faster!
Addition means putting groups of numbers or objects together to find the total amount. We use the '+' (plus) sign for addition. For example, if you have 3 apples and get 2 more, you add them to find the total number of apples.
Subtraction means taking one number or group away from another to find out how many are left, or to find the difference between two numbers. We use the '-' (minus) sign for subtraction. For example, if you have 5 biscuits and eat 2, you subtract to find how many are left.
To solve addition and subtraction problems, especially with bigger numbers, we can use different strategies:\n* **Counting On/Back**: Start at one number and count forwards (for addition) or backwards (for subtraction).\n* **Using a Number Line**: Jump forwards or backwards on a number line.\n* **Partitioning**: Break numbers into tens and ones to make them easier to add or subtract separately.\n* **Known Facts**: Use number facts you already know (like 'friends of 10' or doubles) to help.
Addition and subtraction are 'opposite' operations. They undo each other! If you know an addition fact, you can easily find related subtraction facts. For example, if 4 + 6 = 10, then you also know that 10 - 6 = 4 and 10 - 4 = 6.
Key facts to remember
- 1Addition means combining groups, and subtraction means taking away or finding the difference.
- 2The '+' sign means add, and the '-' sign means subtract.
- 3Knowing your number facts (like doubles or 'friends of 10') helps you solve problems quickly.
- 4You can use strategies like counting on, counting back, or partitioning to solve problems.
- 5A number line is a helpful tool for visualising addition and subtraction.
- 6Addition and subtraction are inverse operations; they are opposites.
- 7When adding or subtracting two-digit numbers, it's often easiest to work with the ones first, then the tens.
Worked examples
Example 1
What is 8 + 5?
Answer
13
This is a number fact you should try to remember!
Example 2
Calculate 34 + 23.
Answer
57
This strategy is called 'partitioning'.
Example 3
Find the difference between 68 and 35.
Answer
33
Remember to always subtract the ones first, then the tens.
Common mistakes
- ✗Confusing the addition (+) and subtraction (-) signs in a problem.
- ✗Counting incorrectly when using 'counting on' or 'counting back' strategies (e.g., missing a number or counting the starting number again).
- ✗Trying to do all calculations in their head for larger numbers instead of using a strategy like partitioning.
- ✗Forgetting to add or subtract both the tens and the ones when breaking numbers apart.
- ✗Not checking their answer to see if it makes sense.
Exam tips
- ★Always read the question carefully to understand if you need to add or subtract.
- ★Use a number line, your fingers, or draw pictures to help you count if you are unsure.
- ★Practise your number facts every day so you can recall them quickly.
- ★Check your answer by doing the opposite operation (e.g., if you added, subtract to check your work).
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