Thinking Addition Facts to 18 to Subtract

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The students will be able to find the "difference" in subtraction facts by determining the related addition fact, within 18.

Big Idea

Students apply their understanding of addition to solve related subtraction problems.


10 minutes

The lesson opens with a turn and talk about some of the ways that to use addition to help with a subtraction problem.  

If necessary, guide the students to thinking about how they have learned about fact families, how they've learned about how doubles facts and addition sentences can help them with subtraction.  Explain to the students that today they are going to learn how they can use addition facts with greater numbers to help them with subtraction.

Write on the board the number sentence 12 - ___ = 7 and 7 + ___ = 12.  Explain to the students that the same number needs to be used to fill in each blank.  Have the students turn and talk about what number they think would make sense in both blanks.

Develop the Concept

20 minutes

Students put 16 counters in a row on their desktop, yellow side up.  Write 16 - 7 on the board.  Students are told to flip over the first 7 counters to show the subtraction problem.  

Similar to the activity in the previous lesson, students look at the model to see what related addition problem they could use to help them solve the subtraction problem.  If necessary, guide the students to think about how the 7 yellow counters and the remaining red counters are both parts, and the whole is the 16 counters we started with.  They should be able to see that 7 + 9 = 16.

Students continue to use the same strategy to practice with the following problems:

  • 14 - 6 =
  • 11 - 8 = 
  • 13 - 5 =


Practice the Concept

20 minutes

Students continue to use their counters to create models for the problems on the Thinking Addition Facts to 18 to Subtract Practice.  Remind the students that they must build the concrete model in order to help develop the mental model. 

The goal is for students to practice using a concrete model (chips), so that they can later visualize the picture or movie in their mind when they need to solve similar problems in the future.  

It is helpful to assess student understanding, and make adjustments to practice, during both the Develop and Practice the Concept Sections to meet all students' needs.  If necessary, students meet with the teacher in small groups to complete the steps together and discuss their thinking.


10 minutes

Have the students come back together as a class and have them turn and talk about how they know which addition fact to use when completing a subtraction problem.  The CCSS require students to have an understanding of how numbers work together and how addition and subtraction are related. I think it is important to have students understand that this is the goal of these lessons and to discuss how they are working towards these goals.  Have the students talk about how they could use this to help them build their math fact fluency.  The idea of fluency is something that we discuss often in our class, in both reading and math.  It is important for students to understand that the CCSS suggest that students have opportunities to develop their fluency of basic facts.