Operations & Algebraic Thinking
Understanding Subtraction
Subtraction as taking apart and taking from.
Prerequisites
- Understanding Addition
Taking a Group Apart
Addition put groups together. Subtraction does the opposite — it starts with one group and takes part of it away, leaving a smaller group behind.
Definition — Subtraction
Like addition, subtraction has two everyday meanings. 'Taking apart' means splitting one group into a part that leaves and a part that stays, like 8 cookies with 3 eaten. 'Taking from' means starting with a group and removing some of it, like 8 balloons with 3 popping.
Worked Example — Taking Apart
Worked Example — Comparing Two Groups
Tip
Common Mistakes
Counting backward the wrong number of times, such as counting back 4 times instead of 3 when solving 8 − 3.
Match each backward count to the number being subtracted, one at a time — for 8 − 3, count back exactly 3 times: 7, 6, 5.
Mixing up which number is the starting group and which number is being taken away, especially in a word problem.
The starting amount always comes first. In 'there were 8 cookies, 3 were eaten,' 8 is the starting group and 3 is the amount taken away, so the problem is 8 − 3.
Key Takeaways
- Subtraction takes part of a group away or compares two groups.
- The answer to a subtraction problem is called the difference.
- Addition and subtraction undo each other, which makes addition a good way to check a subtraction answer.
Summary
Subtraction takes a group apart or compares two groups to find the difference. With both addition and subtraction understood, the next lesson uses them together to solve real word problems.
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