Addition and subtraction within 20: Year 1 Maths topic guide

What students learn in AC9M1N04

This unit helps students build a clear, usable understanding of Addition and subtraction within 20. The goal is not to memorise one answer pattern. Students should be able to explain the idea, recognise it in a new example and apply it in a short practice or worksheet task.

• Start with the main idea: add and subtract numbers within 20, using physical and virtual materials, part-part-whole knowledge to 10 and a variety of calculation strategies.
• Use concrete examples, pictures, oral explanation and short written responses before moving to independent practice.
• using drawings, physical and virtual materials and number combinations within 10 to add and subtract collections to 20 adding and subtracting numbers within 20, using a variety of representations and strategies such as counting on, counting back, partitioning and part-part-whole knowledge of numbers to 10; for example, using partitioning and combining 7+5=7+3+2=10+2=12
• Finish with mixed questions so students must choose the correct strategy rather than copy the last example.

Curriculum coverage and elaborations

The content description and elaborations below show the curriculum ideas taught in this unit. Items marked as teaching context support lesson planning but may not appear in the initial eight-question activity.

• Content description: add and subtract numbers within 20, using physical and virtual materials, part-part-whole knowledge to 10 and a variety of calculation strategies
• E1: using drawings, physical and virtual materials and number combinations within 10 to add and subtract collections to 20
• E2: adding and subtracting numbers within 20, using a variety of representations and strategies such as counting on, counting back, partitioning and part-part-whole knowledge of numbers to 10; for example, using partitioning and combining 7+5=7+3+2=10+2=12
• E3: developing and using strategies for one-digit addition and subtraction based on part-part-whole relationships for each of the numbers to 10 and subitising with physical and virtual materials; for example, 8 and 6 is the same as 8 and 2 and 4
• E4: representing story problems involving addition and subtraction of numbers within 20 using a Think Board, recognising and using + and - symbols and the equal sign to represent the operations of addition and subtraction; showing and explaining the connections between any materials used using the language of plus and minus, and the numbers within the story problem
• E5: creating and performing addition and subtraction stories told through First Nations Australians’ dances (teaching context)

How to use this unit

Read the topic guide, use the teacher slide for instruction, then complete the Worksheet, Practice and Test. These three activities use the same eight-question unit bank.

Common mistakes to watch for

• Rushing to a rule before checking the concrete model, drawing or number sentence.
• Using the correct answer once but not being able to explain why it works.
• Mixing up similar vocabulary such as more/less, before/after, longer/shorter or equal groups/sharing.

International curriculum mapping

This table gives closest-topic mapping for search and planning. The Australian Curriculum code is exact; overseas entries are broad equivalents because each jurisdiction structures outcomes differently.

Related Year 1 Maths topics

• AC9M1N01: Numbers to 120
• AC9M1N02: Partitioning tens and ones
• AC9M1N03: Skip counting and equal groups
• AC9M1N05: Additive problem solving and money
• AC9M1N06: Equal sharing and grouping
• AC9M1A01: Skip-counting pattern sequences
• AC9M1A02: Repeating patterns
• AC9M1M01: Comparing length, mass, capacity and duration

Official curriculum references

• Australian Curriculum Version 9.0
• Victorian Curriculum F-10
• NSW Curriculum
• Common Core State Standards
• Next Generation Science Standards
• England National Curriculum
• New Zealand Curriculum
• NCERT textbooks