Strategy

Students can:

• Use inverse operations to assist with mental computation
• Apply problem solving strategies such as working backwards to find missing numbers

Students in Stage 2 should be able to flexibly use both addition and subtraction to solve number sentence problems. Problems that involve missing numbers focus on students using the working mathematically process of problem solving, particularly applying strategies.

Activities to support the strategy

True or False?

Students need to develop the concept of seeing the equals sign as meaning ‘the same as’. That the values on both sides of the equals sign are equivalent. Students also need to understand that you can operations on either side of the equals sign.

Provide students with a number of number sentences, for example,

• 39 – 12 = 15 + 11
• 45 + 11 = 62 – 1

Ask students if the sentences are true. Why or why not?

Ask students to design number sentences using addition and subtraction as operators. Students then swap them with a partner and work out which ones are true and which ones are false. Students need to write/ discuss their reasoning for why.

As an extension, provide students with similar number sentences where one of the values is missing. Ask the students to make the sentence true by adding in the missing number. Again, students should discuss their strategies for solving the problems.

Example: ____ + 36 = 70 – 10

References

Australian curriculum reference

ACMNA083: Use equivalent number sentences involving addition and subtraction to find unknown quantities

NSW syllabus reference

MA2-8NA: Complete number sentences involving addition and subtraction by calculating missing numbers, for example, find the missing numbers ___ + 55 = 83

Online resources

• NRICH enriching mathematics -which symbol?
• Investigating Number Relationships in the Talking about patterns and algebra resource.
  • which of these are true? (page 95) and
  • investigating addition and subtraction facts (page 100).