PLENTIFUL POMEGRANATES [DAY 4]
SUBTRACTION STRATEGY: PLACE VALUE TO ALGORITHM
Explore efficient strategies to subtract values within 50
Intentionality
Math Talk
Visual Math Talk Prompt #1
Visual Math Talk Prompt #2
Visual Math Talk Prompt #3
Visual Math Talk Prompt #4
Visual Math Talk Prompt #5
Visual Math Talk Prompt #6
Purposeful Practice
Resources & Downloads
Educator Discussion Area
Intentionality & Unit Overview
Length of Unit: 5 Days
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Students will solve problems that require regrouping to solve.
Intentionality…
The purpose of the Day 4 activities is to reinforce key concepts from Day 3. Students will engage in math prompts and will have an opportunity to complete purposeful practice. The math talk and purposeful practice serve to develop a deeper understanding of the following big ideas.
Some of the big ideas that may emerge through this task include:
- Numbers can be decomposed by separating a whole into two or more parts
- Exchanging or regrouping occurs when the subtrahend in a place value is bigger than the minuend
- Subtraction names the missing part in terms of the whole
- Different subtraction situations will elicit different strategies
- Number relationships provide the foundation for strategies to help students remember basic facts
- Subtraction can be used in either take away, comparison, or missing addend situations. This task will explore the take away method.
- Models can be used to connect concrete to abstract
Math Talk
To practice using a place value strategy, have your students work through these problems one at a time. As you present each problem, provide students with some quiet, individual thinking time during which they can solve the problem and show their thinking on a dry erase board, paper, etc. Then have students share their thinking either in pairs or with the whole group. As you facilitate these conversations, notice how students are recording their thinking and use purposeful language to develop an understanding of the exchange/regroup process. As the questions change from base ten manipulatives to written ones, watch for students reversing the numbers instead of exchanging them.
Reversals are a common misconception when using an algorithm. Students will subtract the smaller value from the bigger value regardless of which one is the subtrahend or minuend. Additional use of the base ten manipulatives may be necessary to demonstrate why an exchange needs to occur.
The numbers in the problems are specifically chosen to elicit the place value strategy over an alternative strategy. Students are always encouraged to be flexible with the strategies they use, so an alternate strategy is not to be discouraged. Instead, honour that strategy with a student but choose students to share who are using the place value algorithm.
Manipulatives may be necessary for students to “see” how the exchange process works. For this purpose, base ten blocks are used in the first three prompts but then replaced with number expressions. The final prompt uses numbers that are above 50 to allow students to subtract more in the tens column.
Prompt 1:
33 – 12
Prompt 2:
41 – 15
Prompt 3:
43 – 19
Prompt 4:
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Prompt 5:
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Prompt 6:
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Visual Math Talk Prompt #1
The first prompt does not involve regrouping or exchanging. This is to warm students up to subtraction. Watch if students are solving this question from left to right or right to left. Do they subtract the tens first or the ones first?
Facilitator Note: The base ten blocks are used in these prompts as a visual to demonstrate the amount being removed. The action of removal may need to be demonstrated in “real time” to assist some students. Consider providing base ten blocks to some students or demonstrating as a group.
Students can see what has been removed from which place value.
Visual Math Talk Prompt #2
This prompt will involve regrouping/exchanging to occur. The base tens are used in the prompt to support the discussion of the exchange. Encourage students to say that they exchanged a ten from the tens place value column to the ones place value column. This is why 1 turns into 11.
The visual demonstrate what is happening when a ten needs to be exchanged into the ones place value. Now that there are enough blocks in the ones column, the subtraction can be done.
Visual Math Talk Prompt #3
Similar to the second prompt, this prompt requires an exchange to occur. This is the last prompt to use base ten manipulatives as a visual. Continue to generate discussion using the appropriate language to match the process.
“We are exchanging a ten from the tens place value column into the ones place value to make 3 turn into 13. Now there are enough singles to remove/subtract”.
Visual Math Talk Prompt #4
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Visual Math Talk Prompt #5
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Visual Math Talk Prompt #6
Purposeful Practice
Small Group Game
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Small Group Assessment Opportunity
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Resources & Downloads
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Printable Lesson Plan PDF
Videos, Images & Media Files
Apple Keynote Presentation
Powerpoint Presentation
Printable Consolidation Prompts
Educator Discussion Area
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