On Your Mark...

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SWBAT model missing addend and subtrahend number sentences using a number line.

Big Idea

The number line is a tool students will use in math from Kindergarten all the way through high school! This lesson has students explore how to model missing addend/subtrahend using the number line.

Setting Up the Learning

5 minutes

The number line is the focus in this lesson, and, if you click on my reflection, you can see how this lesson supports number line understanding along with future standards within the Common Core! This lesson is aligned to MP5-Use appropriate tools strategically. 


We have been learning all about how to solve missing part number sentences. Today we are going to look at how we solve these on a number line.


This is important because the number line is an incredibly important tool! You will use it in math all the way to high school, so it is important you understand how to use it to solve math problems.

Objective: How can I model a missing part number sentence using a number line?

Opening Discussion

10 minutes

The whole opening discussion is about the number line. CCSS MP5 says, "Use appropriate tools strategically." Our discussion will be centered around how students use the number line, where they find their answer, etc. This prepares kids to use this essential tool on their own!

Start with: 14

Get to: 11

I'll have students show me where to start and where to stop. I'll also have them tell explain why we went backwards/forwards on the number line. 

After we show this on the number line, guiding questions:

  • Would I use a + or - in my number sentence? Why?
  • What number will I write first? How many did I need to take away? How many did we end with? 

Now let's look at a number sentence: 12 + ____ = 16

  • Where will I start? 
  • What am I trying to get to?
  • Am I going to count forwards or backwards?
  • How will we figure out what goes in the blank?

I'll model this on the class number line. After we are done modeling, I'll have students come up and show me what number goes in the blank and how they know.

Guided Practice

15 minutes

Students get paper number lines with a pencil or get laminated number lines with white board markers.

With each round, I'll give students a number sentence. I'll have students figure out what the number sentence tell me about where I should start and where I need to get to. You can get free number lines here

Number Sentence: ______________

Start with:___________

Get to :____________

After each new number sentence, I'll have students model this on their number line and then share it with their partner.

Guiding Questions:

  • Did you partner add/subtract? Why did they do that?
  • Do your number lines look the same?
  • Why did your partner jump forward/back? Why did they start there/stop at this number? 
  • What story problem could you make up to go with this number line? Would it be an adding story or a taking away story?
  • What number goes in the blank? What is the missing part? 

Intervention, either for small group or whole group:

For students who are struggling with this concept, you can give them a cube train to use first. Letting them work with cubes first and then model on the number line gives them a scaffold because they already know the answer! This is a nice way to introduce a new, important tool. Example: "I am starting with 10 cubes and need to get to 14 so I am going to get more cubes out. How many cubes did I need? How many jumps did you make?”

Independent Practice

15 minutes

Students play start with/get to at their desks. Each problem has a number line for them to model on and an equation to figure out.

Group A: Intervention

This group needs to model with cubes before they try the number line. I'll have these students show the equation with cubes first, then show what they did on the number line. Numbers under 10.

Group B: Right on Track

Students model on the number line and solve equations. Numbers to 20

Group C: Extension

Students model on the number line and solve equation. The extension is that students fill in their own number lines for numbers to 50. They fill in the numbers they need to model the number sentence.


5 minutes

We will do one more round whole group and I'll have students complete an exit ticket! The exit ticket just has students solve 1 missing addend and 1 missing subtrahend equation. They will not be given a number line on the paper, but they will be available in the classroom. Students can opt to choose any strategy, and they will have space available to show how they solved.