Diane Siekmann SUNSET CANYON SCHOOL, PHOENIX, AZ
3rd Grade Math : Unit #2 - Subtraction : Lesson #2

Break It Apart

Objective: SWBAT to subtract mentally with regrouping using place value strategies.
Standards: 3.NBT.A.2 MP2
Subject(s): Math
60 minutes
1 Warm-Up - 5 minutes

First I set the context for this lesson by explaining to the students that people don’t always have paper and pencil and time to write down numbers and subtract to find out the difference between numbers.  We have to use what we know about adding and place value to help us subtract. 

I ask the students to talk to a partner or their group to discuss the following questions: 

What are some instances when we would need to subtract in our head?

Why do you think this is something we need to learn in third grade?

What did you learn about subtraction in second grade? 

3.NBT.1 states that by the end of third grade, students will be able to add and subtract fluently within 1000 using strategies and algorithms.  This lesson provides an opportunity for students to apply a place value strategy of using friendly numbers in subtraction with regrouping. 

2 Demonstrate The Idea - 3 minutes

Using unifix or interlocking cubes, I demonstrate a subtraction problem because I know this is a new strategy for the students to grasp, and having a visual will provide them with a reference to this strategy. 

First, I select a number with tens and ones, such as 45.  I ask the students how I would build that number with the unifix cubes.  My students are able to explain that I need four groups of ten and a five individual cubes.  Through discussion I may say these cubes represent pencils or cookies, and that I need to share some of these with another person or class. 

Next, I explain that the class next door needs 18, and we will be sharing.  I ask the students to think about how I will be able to give away 18.  

This leads to the idea of having to break apart a 10.  When they reach this point, I ask the students to consider that even though I'm breaking apart a group of 10 cubes, I still have the same number at this point.  

I demonstrate how breaking apart the 18 into a group of 15 and 3 ones will help us to subtract easily by matching the ones digit in the whole amount.  It is important to call attention to the ones digit, and I underline it for visual learners.  

The number sentences are listed here for clarity:

45 - 18 = _____

(45 - 15) - 3 = _____

30 - 3 = 27

 

This would also work using 5 and 13 to break apart 18.  The number sentences would look like this: 

45 - 18= _____

(45 - 5) - 13 = _____

40 - 13= ______

(40 - 10) - 3 = _____

30 -3 = _____

In this example, there is a second breaking apart of 13 into 10 and 3.  It's one extra step, but one may happen as students focus on only matching the ones digit without keeping the tens digit in the first subtrahend. 

3 Anchor Chart - 10 minutes

In this section you will create an anchor chart, titled "Break It Apart", with the students to demonstrate the process of breaking apart (decomposing) a number.  To relate this concept to a concrete idea, I take an old wooden pencil and actually break it into two pieces. (Note: I use a pencil that never seems to sharpen well for this.)  

I share with students that it's still a pencil. I could tape it back together and it would still work as a pencil.

I use the same number sentence from the demonstration with the connecting cubes to write out the chart.  

45 - 18 = _____

Because I know my students retain information if they also write it, I have them build the same anchor chart in their math journal at the same time as we are creating it on chart paper.   

I focus on breaking apart the number by matching the ones digit from the whole amount to allow for mental math subtraction.  In this example using the five in the ones digit as the guide, and breaking apart 18 as 15 and 3, or 5 and 13. I use color coding to focus students on this important step.  Color coding is also an ELL strategy useful for may visual learners.

This strategy is based on the Common Core Math Practice 2, Reasoning Abstractly and Quantitatively, because it supports the students with decomposing the subtrahend to create an easier, friendlier number to subtract. 

4 Guided Practice - 10 minutes

I have students use individual whiteboards to practice this process of breaking apart numbers. Also, I create word problems from these numbers to provide context.  

First, I demonstrate to the students how to underline the ones digit. This helps them to identify which number to break apart (decompose).  

I model each step and number sentence for several problems, pausing and ask students to identify the ones digit, to identify the numbers to use in this strategy, to ask for the next steps, etc. 

Examples include: 

 

Board Example 2 

It can also be written this way

 Board Example 2

I ask students which one has more steps, and we discuss why.  Some students realize that taking away a larger amount at the start saves steps.  Other students focus on only using exact matches to the ones digit in the subtrahend, without considering the tens digit. Right now, both approaches are OK, so long as students understand what we are doing.

5 Try It Yourself - 10 minutes

We are going to use a foldable to recreate the structure of steps that we are taking in decomposing to mentally subtract. I demonstrate each fold, and students follow.  

I fold the paper into thirds along the vertical. (If using 8.5 x 11. If using square paper, which side folds makes no difference.) Some may know this as a "hotdog fold" (Dinah Zike). Using the idea of creating a long, thinner shape like a hotdog bun may help some students follow along. Next, I show students how to turn their paper sideways, and fold the long shape into halves. When it is opened, there are six boxes, defined by folds. A printable example is attached in the resource section. 

Next, I have students use their math journal to rewrite the problem, broken down, in the first square on their paper.  Then, I write and display on the document camera five additional problems, one to be placed into each square, to be solved by using the modeled mental math strategy.  

The students work with their shoulder partner at their desk, or in a group of three when necessary.  My goal is to see if they can apply the decomposing strategy, working through each step to subtract mentally

Link

 

Check for Understanding
Checks for Understanding

Many of the students struggled with this process on the first day.  I think using more examples with manipulatives and breaking apart will support their learning.  This is something I will need to reteach, and enhance for student understanding and success.  About 20% of the students understood the process after the first lesson, and the remaining students work showed their confusion with the process.  I will also review this strategy using a number line. 

6 Review - 5 minutes

I have the students tell me the steps to solve a new subtraction problem.  I connect the review of this process of mental math subtraction by using addition to check some of the problems used during the Try It Yourself section.  

For example:  

45 - 18 = 27 

(27 + 3) + 15 = _____

30 + 15 = 45