Operations & Algebraic Thinking
Solving Word Problems Within 20
Using addition and subtraction to solve one-step word problems.
Prerequisites
- Understanding Subtraction
Choosing the Right Operation
Word problems describe a situation in sentences instead of a bare equation like 5 + 3. The math skill you already have — adding and subtracting — doesn't change. What's new is deciding which operation the story is asking for.
Look for the action in the story. If something is joining, combining, or growing, that's addition. If something is leaving, being eaten, breaking, or being compared, that's usually subtraction. Drawing a quick picture or using objects to act out the story can make the operation clear.
Worked Example — An Addition Story
Worked Example — A Subtraction Story
Worked Example — A Comparison Story
Tip
Common Mistakes
Grabbing the two numbers in a story and adding them without reading what's actually happening, even when the story describes something leaving.
Picture the story happening — are objects joining a group, or leaving it? Let the action, not just the presence of two numbers, decide addition or subtraction.
In a comparison problem, subtracting the numbers in the wrong order, like writing 4 − 9 instead of 9 − 4.
When comparing, always subtract the smaller amount from the larger amount, since the difference can't be a negative number of objects.
Key Takeaways
- The story's action — joining or leaving — decides whether to add or subtract.
- 'How many in all' points to addition; 'how many left' or 'how many more' points to subtraction.
- Acting out or drawing the story helps confirm which operation fits.
Summary
Solving a word problem means reading the story, deciding whether groups are joining or leaving, and then adding or subtracting. The next topic looks closely at the equal sign itself — what it really means for two sides of an equation to match.
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