Finding a missing number in addition
Understand subtraction as finding an unknown addend (e.g. 10 − 8 = ? is the same as 8 + ? = 10)
What a learner can do afterwards
- Solve 10 − 8 by thinking 'what do I add to 8 to make 10?'
- Explain that subtraction can be thought of as a missing-addend problem
- Use known addition facts to solve related subtraction problems
The lesson
When you see a subtraction problem like 10 − 8 = ?, you can turn it into an addition question instead. Ask yourself: what number do I add to 8 to reach 10?
Mia has 8 crayons. She wants 10 crayons. She thinks: 8 + ? = 10. She counts up 2 more, so she needs 2 more crayons. That means 10 − 8 = 2.
You already know your addition facts. Use them backwards! If you know 6 + 4 = 10, then you also know 10 − 6 = 4 and 10 − 4 = 6.
10 − 8 = ? is the same question as 8 + ? = 10, so your addition facts can solve subtraction too.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.