Grade 1 · Remove a part and recount · 441 of 750
Remove a part and recount: starting objects: 18; objects taken away: 3
At a classroom activity, 18 counters are available and 3 are taken away. Find the number left after the removal, explain the calculation, and check a related question.
Try the question, use a hint when you need one, and compare your reasoning with the worked solution. The animation starts with this chapter’s values; changing its controls explores a new case.
01 · Make a prediction
Your practice question
At a classroom activity, 18 counters are available and 3 are taken away. First find the number left after the removal. Then answer: Return one removed object when possible. What remains then?
Use this scratch space or work on paper. Your notes stay on this page and clear when you leave. Answers are for self-checking; they are not automatically graded.
A starting point
Subtraction separates an original whole into the removed part and the remaining part. The remaining quantity must be counted after the removal, not by recounting the crossed-out objects.
Work through the reasoning
Step 1
Identify the quantities and the task
At a classroom activity, 18 counters are available and 3 are taken away. The first task is to find the number left after the removal. Subtraction separates an original whole into the removed part and the remaining part. The remaining quantity must be counted after the removal, not by recounting the crossed-out objects.
Step 2
Calculate the original result
18 − 3 = 15. At the end, 15 remain. Adding back the removed 3 reconstructs the original 18.
Step 3
Complete the follow-up calculation
Return one removed object when possible. What remains then? 16 remain: 18 − (3 − 1) = 16.
The answer
18 − 3 = 15. At the end, 15 remain. Adding back the removed 3 reconstructs the original 18. Follow-up: 16 remain: 18 − (3 − 1) = 16.
Compare the method as well as the result. A different valid method may reach the same answer. Keep exact values until the last step when the question asks for rounding.
02 · Explore the model
See the mathematical relationship move.
Use Play, Pause, and the timeline to inspect the construction. Reset restores the question’s original settings. The displayed assumptions describe where this model applies.
Watch the relationship
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From experiment to screen.
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The mathematical idea
At a classroom activity, 18 counters are available and 3 are taken away. Subtraction separates an original whole into the removed part and the remaining part. The remaining quantity must be counted after the removal, not by recounting the crossed-out objects. At the end, 15 remain. Adding back the removed 3 reconstructs the original 18.
18 − 3 = 15
03 · Reflect and transfer
Explain what changes and why.
Move the timeline to increase the removed amount. The pale or crossed-out section records what has left; the colored section records what remains. Explain which quantity changes and which relationship stays valid. Use the starting values again before checking your written answer.
This is a distinct guided scenario using a reusable mathematical model. Similar-looking diagrams can represent different given values and conclusions; they are not different mathematical theories.