Primary 1 maths · Operations · About 8 minutes
Recall addition and subtraction facts within 20
Check a building block, predict, try the model, then explain and use the idea.
Start the interactive lesson →Open practice activity
What you will learn
- Recall addition and subtraction facts within 20
Models used: number sentences, stories, diagrams.
Before you start
Replay what taking away means
There are 13 counters. We take away 5. How many stay?
Our starting group has thirteen counters.
Let’s replay your change. Here are the same 13 counters before any are taken away.
Blue counters stay. Crossed counters have been taken away.
A cross means taken away. It does not mean an extra counter.
Cross out five. Those five no longer belong to the group that stays.
Blue counters stay. Crossed counters have been taken away.
Count only the blue counters without a cross. Eight stay.
13 − 5 = 8
Blue counters stay. Crossed counters have been taken away.
Blue: the eight that stayed. Orange: the five put back.
Put the five back to check. Eight that stayed and five put back make our starting thirteen.
8 + 5 = 13
What did the picture help us see?
Put back what we took away. Check: 8 + 5 = 13.
Worked example: A new example, with your turn
Start with 17 counters. Take away 3. How many stay?
Blue counters stay. Crossed counters have been taken away.
Start with 17 counters. This time we will take away only three.
Blue counters stay. Crossed counters have been taken away.
Three are crossed out. You count the counters that stay before we show the total.
Blue counters stay. Crossed counters have been taken away.
Start with 17 counters. Take away 3. How many stay?
14
Fourteen counters stay. The subtraction sentence records the change you counted.
17 − 3 = 14
Blue counters stay. Crossed counters have been taken away.
Blue counters stayed. Orange counters are the three put back.
Put the three back. The two parts rebuild our starting seventeen.
14 + 3 = 17
Common mistakes to discuss
- Counting from one every time. Known facts should be recalled, not rebuilt. Using 8 + 2 = 10 to reach 8 + 5 is faster and less error-prone than counting five steps.
Check your understanding
- show what taking away does to a group
- connect an addition fact to its subtraction fact
- find a missing part and check the whole
Explain your reasoning to an adult, then try a fresh question in the interactive lesson. A result from one attempt is not proof of lasting mastery.
Try this lesson →Related Primary 1 lessons
- Find the missing partFind a missing part when the whole and one part are known · About 10 minutes
- Missing numbers in take-awayFind the starting number when the parts are known · About 10 minutes
- Add by combining two setsAdd by combining two sets · About 8 minutes
- Subtract by taking awaySubtract by taking away · About 8 minutes
- Connect addition and subtraction with number bondsConnect addition and subtraction with number bonds · About 8 minutes
- Use +, − and = correctlyUse +, − and = correctly · About 8 minutes