Think Addition to Solve Subtraction

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SWBAT use addition facts to solve subtraction equations within 20.

Big Idea

My first graders previously learned the relationship between addition and subtraction through fact families. Now I want them to use this relationship as a tool to solve for unknown differences.

Rev Them Up

5 minutes

I will have my students play "Cross the line." Students will line up facing me. I will use masking tape to create a line on the carpet. I actually leave the tape on the carpet because I use this game every so often. The kids really like it. I will say, "Cross the line if ___________." If the students agree, they cross the line to the other side. If they do not, they don't move. Check out this video where I used it another lesson practicing addition. Any student who crosses the line and should have stayed put, has to sit down. You can make each advancing problem more difficult to eliminate more players or keep them steady and have a good review.


  • Cross the line if 10-5=5. (They should step over.)
  • Cross the line if 8-6=4. (Stay put.)

One good method to make sure your students understand an activity or game is to ask for a volunteer to repeat the instructions to you. If anyone did't pay attention to you or wasn't able to process the directions the first time around, this gives them another opportunity to hear what is about to happen.

Whole Group Interaction

15 minutes

My students have learned about the inverse relationship between addition and subtraction in this previous lesson on fact families. The inverse relationship between addition and subtraction can be used to solve subtraction equations; every subtraction fact has a related addition fact. This relationship is very important to teach because the CCSS expect students to identify and use this relationship as a strategy for computation. (1.OA.C.6). This strategy allows students to reason abstractly and quantitatively by applying the connection between addition and subtraction (MP2).

I will use this activity to review fact families and refresh my student's memory of the amazing connection numbers have with each other. It is close to winter break in my district, so to increase their motivation and attention, I am using Christmas trees to write our fact families on that we create. Here is pic 12, and 3. You can definitely change your recording tool to any shape or object that would work for you.

My students sit in groups of 5 during this part of the lesson, and I can use these small groups to encourage collaboration and discussion. I will be asking several questions, but I want my students to discuss with each other before I take answers. I will pose a question and ask my groups to talk among themselves and decide on their answers. This provides an opportunity for every student to participate. I will begin with asking:

  • What equations can I create from 3, 5, and 8? (Answer: 3+5=8, 5+3=8, 8-5=3, 8-3=5; I would put this all on one tree.
  • (Write 7+1=8 on a tree.) What problems could I write that are related?
  • (Write 8-2=6 on a tree.) What problems are related to this problem?

This discussion should provide a good review for them to use for the next part of this lesson.

Independent Practice

10 minutes

I have had several mentor teachers give me sage advice over the years and this advice has slowly evolved and melted together and has made me what I am today.  The best advice I was ever given was simple, "they will learn in spite of you." So even on a bad day, when a lesson has not gone my way, or misbehavior has thrown us entirely off track, or schedule changes flip my plans upside down, this advice comforts me.  First graders are sponges, and I know that I can pick back up tomorrow and everything will be okay. Remember this as you teach and don't sweat the little stuff and let it build up to force you into giving up.  There's always tomorrow, and I have had many "do-over" lessons.

This advice is good to keep in mind especially during the holidays when lessons go off the rails all the time!

Need: Cut lots of 1" strips of green and red paper. And possibly be prepared to cut more in the middle of your lesson. At least 3 index cards per student.

Now back to what we are here for... it is not always easy to incorporate a craft activity into a math lesson, but I plan to do exactly that.  I will have them build a model for an addition problem and then identify the subtraction facts that are related to this model. 

First, I will write the problem 2+3=5 on one side of an index card. Then I will be building a model using 2 red strips and 3 green strips to build a model of 5 total. I will build the links and hook them together one color at a time. Then show them how the 2 red combined with the 3 green equal a chain of 5. Now I will ask them to help me figure out what subtraction problems are related to this equation. I will turn the card over and write what they say, which should be 5-3=2 and 5-2=3. Watch this model video and look at the completed chains.

Now I will let them pick their beginning addition model and construct their own model. It will be very important to walk the room and support each student in this process. Some will finish fast and I am going to encourage them to build more and to increase in complexity. 

Lesson Extension

5 minutes

You can see from the videos in the resource section that the kids became really excited about this activity and I want to take advantage of that excitement. I am going to alter the chain link activity and provide a great opportunity for more practice to my students. I have some students who are fast workers during math time and this activity will be perfect for them to work quietly and still be engaged learners. I will add a third color to the link activity and place the materials in a center.  I will show them an example of adding three numbers together. Common core pushes our first graders and challenges them to solve rigorous equations involving adding 3 whole numbers. (1.OA.A.2). This center will help in building this skill in a fun way. I will emphasize that not only are they constructing the model, but they must write the equation they create.