Make a Ten, Subtraction Style

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Objective

The students will be able to use the strategy of "make a ten" to solve basic subtraction facts.

Big Idea

The big idea of this lesson is that some subtraction facts can be solved by subtracting from the larger number an amount to get ten and then subtracting the remaining amount.

Activator

10 minutes

I begin the lesson by having students turn and talk about how they can make a ten to help them add.  If necessary, guide the students to think about how they learned about making a ten when adding three numbers  and how they learned to make a ten to help them by taking the chips from one ten frame and making a full ten.  It may be necessary to have the students use their double ten frame mat to help them recall the strategy.

Develop the Concept

20 minutes

"How could you could show the number 13 on your Double Ten Frame mat?"  

Students set up the full ten frame (building should always begin with bottom frame, upper left hand corner and the bottom frame is built first as it represents the "first" ten) and then place 3 chips on the second ten frame.  If they struggle with this skill, have them practice building numbers within 20 on their double ten frame, as it will be necessary for the students to be able to accurately do this in this lesson. Here are some aspects of 10 frames that are important for students to "notice":

  • You don't need to count to know how many, because:
    • All spaces filled is 10.
    • 1 row of spaces filled is 5.
    • 1 row plus 1 more is 6.
    • 2 empty spaces is 8
    • 1 empty space is 9
    • 3 empty spaces is 7
  • Two ten frames make 20
    • All spaces filled is 20
    • 1 filled frame, plus 1 half filled frame (1 row) is 15

Write on the board the following word problem:

There are 15 frogs on lily pads in the pond. 7 frogs hop of the lily pads.  

Ask,  "What number sentence could you use to solve this problem?" (15 - 7 = __)  

"Show this subtraction problem using your double ten frame."  

If they have difficulty it may be necessary to prompt the students to think about the number that is the whole (15). And then restate the question, " There are 15 frogs on lily pads in the pond. What happens next?"

Your goal is to prompt students to reason this out for themselves, rather than directing them to "take away", so avoid using language that indicates what students should do (such as, how many are left).

"So, where are we starting our problem?           What happens next?"

The goal is to have the students realize that they begin with the whole, and remove "frogs".

Once students have creating the problem, write it  the problem on the board, 15 - 7 = ___.  

Next have the students look at their double ten frame and think about how they could "Get back, get back, get back to ten." (Get Back to Ten Chant)  This means that the students should be thinking about how many chips should be removed for them to get back to just a full ten frame.  

Have the students take their ten frame and "get back to ten" by removing the 5 chips on the second ten frame.  Then ask the students if they removed all 7 the way the number sentence asks them to.  They should be able to tell you that they only removed 5 and need to take two more in order to remove the correct amount.  Have the students think about a new number sentence for what they have left to remove.  They should think about 10-2, the whole is now the full ten frame and they still need to remove 2 more.

Example of Making Ten

Continue to guide the students practice with these skills with the following problems:

16 -9

13-5

18 -9

You can always give the students more or less practice as you assess the students' ability to solve the problems.

 

Practice the Concept

20 minutes

As I shared in my reflection, I believe that there can be - and should be - joy in learning. There are many ways to make math practice feel and look like a game, and still be a rigorous and CCSS aligned activity.

I assign the students as partners to work on a Practice Page but include a "game-like" twist of using a die to determine the number of the problem to solve.  They are expected to also draw a model of their strategy.

It is helpful for the students to work in partners so that they may help solidify each other's understanding and they can work together to build and solve the problems.  It is important for the students to practice building each problem using the double ten frame mat and counters.  The goal is for the students to continue to use the concrete model so that eventually, when they develop that understanding, they will be able to see the mental model.

Summarizer

10 minutes

Have the students come back together as a class and share some of the problems that they solved and how they solved them.  As the strategies are shared, ask the class if they could use this strategy.  The CCSS require students to develop mental math skills, as they become developmentally appropriate.  For students in second grade, much of their work with be done with hands-on activities to begin to bridge students to visualization of mental models that they can use in recalling basic subtraction facts fluently.