This activity is designed to encourage students to develop and explain their own strategy to solve subtraction problems and to then use an alternative strategy to check if their calculations are correct. It is also intended to expose students to the count up to strategy which allows students to use addition to solve subtraction problems.

##### Purpose
• Recall addition facts and related subtraction facts
• Recognise the relationship between addition and subtraction
• Use knowledge of addition, subtraction to solve more challenging problems
• Recognise that subtraction is not commutative
• Partition numbers to assist with calculations
• Identify whether a strategy is efficient
• Explain strategies using a written method
• Recognise that written equations must balance
• Check solutions using an alternative strategy

##### Year 3 – Numbers & Place Value
• Recall addition facts for single-digit numbers and related subtraction facts to develop increasingly efficient mental strategies for computation(VCMNA133)
##### Year 4 – Numbers & Place Value
• Apply place value to partition, rearrange and regroup numbers to at least tens of thousands to assist calculations and solve problems (VCMNA153)
##### Year 4 – Money & Financial Mathematics
• Solve problems involving purchases and the calculation of change to the nearest five cents with and without digital technologies (VCMNA160)
##### Year 4 – Pattern & Algebra
• Use equivalent number sentences involving addition and subtraction to find unknown quantities (VCMNA163)
##### Year 4 – Time
• Use am and pm notation and solve simple time problems (VCMMG168)

##### At the end of this lesson students should be able to answer the following questions
• Can you recall addition facts involving single-digit number?
• What are the related subtraction facts?
• Can you record the fact family related to a given number fact?
• What strategies can we use to solve subtraction problems?
• How can we check to see if our strategy is correct?
• Why isn’t subtraction commutative? i.e. why doesn’t 3 – 4 = 4 – 3?
• How can we partition numbers?
• What is the relationship between addition and subtraction?
• How can we use addition to solve subtraction problems?
• Is the strategy used efficient?
• How can we check our solution?