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# Introduction to Change Unknown Problems

Lesson 8 of 13

## Objective: SWBAT draw pictures to represent change unknown problems and write number sentences for change unknown word problems.

*45 minutes*

#### Hook

*10 min*

I start class by posting a problem of the day on the board.

**At the Mardi Gras Zulu parade they had seventy nine coconuts. After the parade was over, they only had nineteen cabbages left. How many coconuts did they throw off of the float?**

I divide students into groups and give them each a basket of cubes and a white board. I ask them to solve this problem using their cubes.

As students use their cubes, I circulate to support students and make sure students are on the right track.

Then, using their cubes, I ask students to write a number sentence on their white boards.

As students work, I circulate to determine what strategies students are using and what common mistakes are being made.

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#### Introduction to New Material

*10 min*

*I would like for two groups to share HOW they solved this problem. As your group shares, please show us how you used your cubes to make a number sentence. *

I ask two groups who solved the problem accurately in different ways to come and share. Some students might have solved the problem by subtracting (87 -14 = _____) Others may solve by setting up the problem like this : 14 + _____ 87 or 87 - _____ = 14. Some students may have used cubes, others may have used base ten blocks or tens and ones drawings. Others may have been able to visualize the problem and simply subtracted.

As students share their strategies, I ask them to explain WHY they choose that strategy using guiding questions: (1) *how did you know to set the problem up like that?* , (2)* why did you choose to add/ subtract?*

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#### Guided Practice

*10 min*

*Now you are going to have a chance to work in partners on a word problem. You can solve this problem similarly to how you solved the problem of the day and you will be allowed to use cubes if that helps you to solve the problem. *

As students work, ask guiding questions

*1) How did you know to subtract? OR How did you know to add?*

*2) How does your number sentence represent your picture?*

*3) How did you know to set the problem up like that?*

When students have finished working on the word problem, I bring my class back together and have two or three students who did not share during the introduction to new material share their picture and their work with the class. As they share, make sure that they are explaining WHY they chose to set up their number sentence the way they did.

#### Resources

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#### Independent Practice

*10 min*

Independent practice is differentiated based on understanding of this concept. I determine groups based on performance and understanding during the guided practice and general mathematical understanding.

**Group A: Intervention.**

Students in group A will work independently or in partners to solve change unknown word problems using numbers 30-50. Students in group A will have cubes available to them if setting up the problem with cubes helps them visualize the action in the problem.

**Group B: Right on track!**

Students in group B will work independently or in partners to solve change unknown word problems using numbers 30-100. Students in group B will have cubes available to them if setting up the problem with cubes helps them visualize the action in the problem.

**Group C: Extension**

Students in group C will work independently or in partners to solve change unknown word problems using numbers 30-200. Students in this group will also be challenged to write their own change unknown story problem. Students in group C will not have cubes cubes available to them--they will be challenged to visualize the story problem in their mind.

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#### Exit Ticket

*5 min*

*Today we found and shared our strategies for unknown change problems. Right now, you are going to show me what you know by working on an exit ticket. *

As students work on their exit tickets, I circulate to check for understanding and note students strategies.

If time permits, I go over the exit ticket with students so that they can receive immediate feedback.

#### Resources

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- LESSON 1: What is One Hundred More? What is One Hundred Less?
- LESSON 2: Adding Three-Digit Numbers Without Regrouping
- LESSON 3: Adding Three-Digit Numbers with Regrouping
- LESSON 4: Adding Four Three-Digit Numbers
- LESSON 5: Our Strategies for Result Unknown Problems
- LESSON 6: Two-Step Word Problems (Result Unknown)
- LESSON 7: Solving Result Unknown Problems with Three-digit Numbers.
- LESSON 8: Introduction to Change Unknown Problems
- LESSON 9: How do I solve a change unknown word problem?
- LESSON 10: Acting out Change Unknown Problems
- LESSON 11: Change Unknown Word Problems
- LESSON 12: Change Unknown Word Problems with Number Lines
- LESSON 13: Subtracting Across Zeros