Adding and Subtracting Within a Decade (Small Group)
Purpose: Develop initial mental strategies for addition and subtraction involving adding and subtracting within a decade (e.g., 43 + 5 and 48 – 3) while making connections to knowledge of structuring and relational knowledge to 10
Description: Students solve addition and subtraction tasks involving a 2-digit number and a 1-digit number. Tasks are presented with Mini 10-frames, making connections to written equations.
What To Say and Do
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1
Use Mini 10-frames to present students with a quantity of 35. Ask, “How many dots do you see?”
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2
Screen the mini ten frames and ask, “How many will there be if I add 3 dots?” Put the frame card with 3 dots next to the screen.
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3
Remove the screen to validate student answers. Ask, “How did you know?”
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4
On a white board, write the equation that corresponds with the task, 35 + 3 = 38.
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5
Present students with four full Mini 10-frames and a frame with 6 dots, asking, “How many dots are here?” [46] Screen the frames.
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6
Ask, “How many dots will we have if I remove 3 dots?” Without the students seeing, use a second screen to cover 3 dots on the frame with 6 dots.
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7
Show students the frame to validate their answers. Ask, “How did you know?”
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8
On a white board, write the equation that corresponds with the task, 46 - 3 = 43.
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9
Facilitate student discussion asking for noticings and reflections about the addition and subtraction tasks.
- What did you notice about the numbers that helped you to solve the task?
- If students counted by ones, ask, “Is there a way you can use something you know to solve the task without counting by ones?”
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10
Continue the lesson using a variety of the following addition and subtraction tasks:
- Tasks that support small doubles (e.g., 32 + 2, 64 + 4).
- Tasks that support 5-plus combinations (e.g., 75 + 2, 65 + 3).
- Removing 1-digit numbers to support small doubles (e.g., 56 - 3, 34 - 2).
- Removing 1-digit numbers that support 5-plus combinations (e.g., 48 - 3, 39 - 5).
- Tasks that involve all combinations within 10 (e.g., 52 + 6, 83 + 4).
Variations
Once students are familiar with the activity and have developed verbal-based strategies, transition to posing addition and subtraction tasks in written expressions.
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