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
1 · Read
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.
More to read
Add and subtract within 20: FAQ
Adding and taking away within 20 is a skill you will use every day. When the numbers are small, you can count on, count back, or make a ten to jump fast. Ten frames and number lines are handy tools that show each step. Pick the tool that feels easiest for the numbers in front of you. Subtraction asks what is left, or what is missing. If 8 + 6 = 14, then 14 take away 8 leaves 6. Fact families stick together, so learning one fact teaches you its partners for free.
2.5: Solving Equations Using the Subtraction and Addition Properties of Equality (Part 1)
Equations with a missing number are puzzles with one hidden piece. Addition and subtraction undo each other, so use the opposite move to reveal it. If something plus nine makes fourteen, take nine from fourteen to find the five. Always test your answer by placing it back into the puzzle. When both sides match, the puzzle is solved.
1.3: Add Whole Numbers (Part 1)
Adding whole numbers means joining counts into one bigger count. Line the digits up by place, start with the ones, and carry tens across when a column passes nine. That carry is just regrouping ten ones as one ten. You can check by adding in a new order, since order does not change the total. Quick recall of facts to 20 makes every bigger sum faster.
Adding and Subtracting Within 100 Using Place Value Strategies (Grade 2)
Place value splits numbers into tens and ones, which makes adding and subtracting within 100 much easier. Add the tens first, then the ones, and join the parts at the end. Breaking numbers apart this way keeps every step small and safe. Draw blocks or a number line if the picture helps. The same splitting trick works for take away problems too.
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2 · Watch
به انگلیسیTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
18 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.