# How do I solve a change unknown word problem?

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## Objective

SWBAT develop number sentences for change unknown word problems?

#### Big Idea

This lessons starts with a concrete representation of a change unknown problem and then pushes students to think abstractly and develop number sentences to solve change unknown word problems.

## Hook

10 minutes

I post a problem of the day:

Jill is building with blocks.  She has 24 blocks in her building.  Jack comes along and takes some blocks from Jill.  Jill now has 13 blocks.  How many blocks did Jack take?

In order to understand this problem, we are going to act it out using  our cubes.

I lay out 24 cubes and label them as "Jill's".  I take away enough to leave 13 blocks remaining.  I cover the blocks I took away and label them "Jack's".  How many did Jack take?

Turn and talk:  What number sentence can we write for this problem?

Students might suggest 24 - ______ = 13 since that is how we modeled the story problem, but 24 - 13 = ______ and 13 + ______ = 24 would also appropriate and may make more sense for some students.

## Introduction to New Material

10 minutes

Now, we are going to practice a similar problem of the day.  On your  white boards, solve this problem:

Denaro has 17 pieces of candy.  Kyla takes some of his candy.  Now Denaro has 9 pieces of candy.  How many pieces of candy did Kyla take?

I allow students 2 minutes to draw a picture to represent the problem and then have them share that picture with their teammates.  Then,  I allow them 3-4 minutes to write a number sentence and solve. I observe what strategies they use.

When finished, I have students share their strategies.  As students share their strategies, I ask the following guiding questions:

1)    Why did you choose to subtract?  Why did you choose to add?

2)    How does this number sentence represent your picture?

3)    How does your number sentence match the story problem?

I write the strategies on an anchor chart entitled “Our Strategies for Change Unknown Word Problems.”

## Guided Practice

10 minutes

Now you are going to work in partners  on some practice problems.   You can use cubes to help represent each the problem.

As students work, I check for understanding and ask guiding questions to groups:

1)    Why did you choose to subtract?  Why did you choose  to add?

2)    How does this number sentence represent your picture?

3)    How does your number sentence match the story problem?

When finished, I have students come back together and go over the problem, checking to determine student understanding and any misconceptions.

## Independent Practice

15 minutes

Independent Practice is differentiated based on student reliance on concrete materials and facility manipulating numbers within certain ranges:

Group A (Intervention)

Students will work on change unknown problems with numbers 10-20.  This group will also be encouraged to use cubes to solve their problems.

Group B (Right on track!)

Students will work on change unknown problems with numbers 10-50.  This group will be allowed to use cubes to solve their problems but will be encouraged to think more abstractly and draw pictures or simply write an appropriate number sentence and solve.

Group C (Extension)

Students will work on change unknown problems with numbers 10-100.  This group will be not use cubes--they will be encouraged to think abstractly and draw a picture to represent the problem or to simply write an appropriate number sentence and solve.

## Closing

5 minutes

Today we worked on change unknown problems.  Can anyone share one of our strategies that we learned today?

I have students review strategies for the whole group, discussing how these strategies work to solve change unknown word problems.