Unknown Addend Problems

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SWBAT use the relationship between addition and subtraction to solve subtraction facts.

Big Idea

I CAN is a big statement. Students have been practicing subtraction and we want them to persevere and solve word problems to create their own subtraction number sentences. They will use known addition facts to solve subtraction problems.

Rev Them Up

5 minutes

My goal for this lesson is to teach the strategy that an addition problem can help us solve a subtraction problem (1.OA.B.4).  My students have been developing subtraction sentences for several days now and I do not need to focus today on creating a subtraction sentence.  They have all shown me they understand the concept, and I want to give them more today to challenge them to apply this knowledge. 

I want to see if they know the answer to 3+4=7, then can they also solve 7-3=4 and 7-4=3 because they know how to make 7 out of 3 and 4.  In today's lesson, I plan on focusing on this concept.

I will begin by reviewing our addition facts. I will have my students stand up and make a large circle in the room. I keep a beach ball handy for different activities and today they will use it for us to play "Beach Ball Math facts." I will hand the ball to my starting person and ask them to toss it across the circle to another student, the student who catches it will have to answer an addition fact that I state. If the student is not sure of the answer, any volunteers can raise their hand as a life line and help them by stating the answer. If the student who catches the ball gets their answer correct, he or she will sit after tossing the ball to another person. Once a student sits their turn is up and only those who have not had a turn will remain standing. The game will continue until the last person gets a chance to answer a problem.

Whole Group Interaction

5 minutes

There is a pattern and structure to our number system and we want to begin pointing those elements out to our students early for them to develop a strong foundation in decomposing numbers (MP7).  I want them to focus on the structure and connections between the two related addition and subtraction problems because this will also assist us in a few days when we practice fact families. Not all students automatically see the connection between addition and subtraction, so I will use this lesson to point it out to them and help them see the relationship.

First, I will work from what they know and write the following problem on the board:


Then I will ask:

Who can use a strategy to find the answer? (I write the answer - 7)  Now, what would happen to this problem if I use the Commutative Property?  (flip the first two numbers)  Does the answer still = 7?  (I will write 4+3=), and I will have them tell me the answer. You can go here for my lesson in Commutative Property.

Next, I will write 7-__=4 and ask:

Do the other two addition problems have any connection to this problem? I will point out that this subtraction problem has a 7 and a 4 in it, is there another number used in the addition problems that is not in this problem?  How does knowing the addition facts for this family help us know how many we should take away from 7 to make 4? 

I will continue to guide them through this problem with questions that support them seeing how knowing how to compose/decompose the number 7 with 3 and 4 helps us solve the related subtraction facts.

Then I will do the same thing with the following example:


Independent Practice

15 minutes

Print and copy the Subtration Number sentence match-up activity. 

Students will need their construction paper books pre-cut and stapled together. I am creating flip-books for them to put their work pages into. I used different colors of paper and cut in half short ways (hamburger fold).  I stapled the printable cover on the front of 6 pieces of paper.

If you do not want to do a book format, you could use a large 14x18 piece of construction paper and have them do their matching stories to sentences on it, but I would pre-draw lines to keep students organized and allow enough space to illustrate their problems.

It will help you during this lesson to begin with an organized work space for each student because there will be several pieces of paper to keep up with.  Make sure they clear their desk off and only have necessary supplies.  If any of your students are crammed into a certain space, try to find them a different area of the room to work.

To get them started on the right foot, I will write each subtraction problem from their handout on the board and the related addition fact and have them assist me in filling in the blanks for the addition problem.

After we complete the addition problems, I will instruct them to cut apart and match the problem to its number sentence and glue them on their book page.  Here they are getting started on cutting their problems apart.  I want them to see the connection between addition and subtraction, so the examples are designed to relate the idea that, for example, "I can solve 10-8 by remembering 8+2=10."

After they have matched those 3 problems, I will have them help me draw pictures to illustrate the problem on the opposite page of each problem to strengthen their concrete understanding of the relationship between the examples.  For example, we will draw 8 bats (or circles) and cross 3 out.  (I always encourage my kids to draw circles if they do not know how to draw whatever the item may be for the problem - that way they don't ever waste time or feel discouraged if their "artistic" skills lag behind their math skills).  Look at the picture of one child building her model for a problem. Also, you will see a picture of one model that was built, and a student had to correct their addition sentence to match the part+part=whole.  Sometimes having first graders draw out the problem is the best way to support them in solidifying their understanding of something so complex at this stage.

Page 2 will be a little more challenging because they will be creating the three number sentences and related addition fact themselves.  I will walk around, check on their progress and provide support. Also, I will remind them they need to illustrate their word problem.


5 minutes

I will ask my students to turn to each other and try to explain step by step what we did to solve our subtraction problems today. I will be walking around the room and listening to their conversations to gather data about who is able to articulate our steps and who might need more practice.