Subtract with 0, 1, and 2

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Objective

SWBAT subtract 0, 1, and 2 from numbers within 20 with fluency.

Big Idea

No solving required! Children need automaticity with subtraction problems within 20. We start to build fluency by learning the identity property and getting automatic with simple subtracting 0, 1, and 2 problems.

Rev Them Up

10 minutes

Need: Brown paper sack lunch bag and 10 unifix cubes

I have introduced subtraction in previous lessons, but this lesson begins a unit for my students to help them develop their "tool chest" of strategy ideas for subtraction. Common core standards for first grade provide high expectations for our little ones to not only develop metacognition with mathematical problem solving, but also to become fluent with basic facts to be better prepared for future, more difficult equations. (1.OA.C.6).

I want to get them thinking about take away and I will use this activity to transition into our math time. I will show and have them count the ten unifix cubes before I drop them in the bag. I will explain:

Students, I am going to reach in this bag and pull out a handful of cubes.  I need you to help me subtract to figure out how many are left in the bag. This is no trick bag, it has exactly what you counted in it: ten.

I will then pull a handful out, for example, I will pull out 5 and ask them to help me figure out how many are left in the bag: 5. I will pour out what is left in the bag on a desk where everyone can see and we will check the answer. Then I will write that subtraction fact on the chalkboard: 10-5=5. I will put the 5 back in and pull out a different amount. Each time I pull some out, I will have them solve the problem and write the equation on the chalkboard.

Whole Group Interaction

10 minutes

I want my students to understand and internalize the identity property, which tells us that if a number has zero subtracted from it it will stay the same. I will have to model this and show my students taking zero away is not affecting my beginning number. These numbers represent a quantity and the numbers are used to compute equations. (MP2). I want my students to begin solving these facts through immediate recognition, and we are building that foundation here. I will show them 10 unifix cubes and subtract 0; there will be ten left.  I will continue to present three more problems in which I take zero away. First graders are smart, and this will seem redundant, but it will solidify their understanding of the identity property while giving the few students I have who are still struggling with number sense the time to process this concrete model.

I will pass out paper and have my students make a list with me at their desk as I build one on the board showing all the problems we solve subtracting zero up to 10.

  • 10-0=10
  • 9-0=9
  • 8-0=8
  • etc.

After each problem, I will ask my students to turn to their neighbor and show them with their fingers each problem. This gives every student a chance to show what they understand and me an opportunity to watch and listen.

Now, I am ready to model with them subtracting one. I will follow the same procedure by starting with 10-1=9 and then do repeated problems with other numbers under ten. But I will also use our class number line and the number line on their desks to point out that each time I subtract one, I am just doing one less - just like when we count backwards or count down. I will have them help me create a list of the problems we create of subtracting one from every number up to 10.

  • 10-1=9
  • 9-1=8
  • 8-1=7
  • etc.

The final part of this lesson is for us to solve problems subtracting two from every number up to ten and make a list. I will use the unifix cubes to represent the numbers and subtract. This will help them internalize these facts by helping them make sense. Once they are internalized, students can begin to build fluency. (1.OA.C.6).

Independent Practice

15 minutes

My kids love to play Bingo of any kind. I found this great FREE site www.bingobaker.com that you can create a BINGO card and it will generate random cards based off your own. You are limited to how many you can print unless you become a paying member, but I was able to create 8 different cards, then copy 3 to use for this lesson. I created cards that had the answers to all the subtraction facts if I subtract 0, 1, or 2 from any number up to 10. I hand-wrote my math facts on Popsicle sticks to use as drawing cards for the Bingo game.   You could type yours or print them from different websites that have them ready made. I make my own from the sticks because I use them in a sorting center for my students to practice their math facts.

I will pass the Bingo cards out and counters to use to cover their spaces. I will instruct them to cover their free space. You can select what kind of game you want them to play: solid row, solid column, diagonal, or black out. I will draw sticks and announce the math fact. They must find the correct answer and cover it with a counter. The rest of the game follows traditional Bingo rules.

Here we are playing bingo.


Closure

10 minutes

Need:  Enough paper sack lunch bags for one per set of partners and ten unifix cubes per set of partners and scrap paper.

I will match my students up with a partner to play this game. The game is played just like we did during my "Rev Them Up" activity, but with a partner instead of me. One way to select partners for your students is to have numbered popsicle sticks and relate each number to a student. Grab sticks at random and pair the students together. This will allow for a random selection of abilities. If you have students who will work best with a certain person, eliminate their number from the set and pair them up from the start. Students will take turns grabbing either 0, 1, or 2 from their grab amounts out of their bag and writing subtraction facts on a scrap piece of paper. I will have them complete at least ten problems together, or five each. This activity furthers their opportunity to build their math practice of reasoning abstractly and quantitatively. They must think about what is left in the bag on each turn and reason quantitatively to subtract for a correct answer. (MP2).