Using Doubles Facts for Solving Subtraction

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SWBAT use doubles addition facts to solve subtraction within 20 problems.

Big Idea

I have been teaching my students many strategies to solve subtraction equations. They have learned the inverse relationship between addition and subtraction. Now I want them to see that relationship between doubles facts.

Rev Them Up

5 minutes

This lesson will require the majority of my students to have memorized the doubles addition facts from numbers 1-10. If you have not taught the addition doubles facts, please go here to teach that lesson first.  Also, you will want them to have lots of practice with these facts before they attempt to use them for solving subtraction.

I will call my students over and explain:

Students, we are going to use our doubles poster to review our doubles addition facts.  These pictures can also help us remember the answers to doubles facts. I need you to help me figure out which doubles facts fits each picture.

Ideas I want them to note:

  • Eyes: 1 eye + 1 eye=2
  • Butterfly: 2 wings+2wings=4
  • Ant: 3 legs+3 legs=6
  • Spider: 4 legs+4 legs=8
  • Hands: 5 fingers+5fingers=10
  • Eggs: 6 eggs+6 eggs=12 eggs
  • Calendar: 7 days in a week+7 days in a week=14 days
  • Crayons: 8 crayons on top row+8crayons on bottom row=16 crayons
  • Semi-truck: 9 wheels+9 wheels=18

I will spread the equations and sums out on a nearby table and ask different students to come up and pick matching pieces for the poster.

Whole Group Interaction

15 minutes

I will copy the addition doubles facts in a column on the chalkboard beginning with 1+1=2 and ending with 10+10=20. Next to this list I will list several subtraction problems and propose the question:

What can I use to help me solve these problems?

I have been teaching my students several strategies, so I expect to receive multiple answers, such as use our fingers, number line, objects, etc.

I will guide them towards looking at the chalkboard and ask them if there is anything there that can help. I have already taught the inverse relationship between addition and subtraction and expect that with a few hints they will see the connection between the doubles addition facts and being able to identify answers for the subtraction facts. The common core standards want first graders to build fluency in solving subtraction facts, but on the road to building fluency, the standards want my students to build a toolbox of strategies to add and subtract within 20. (1.OA.C.6). Here we are connecting our prior knowledge of addition doubles to solve subtraction equations.

Independent Practice

10 minutes

Need: Print the Subtraction Doubles Worksheet and copy for each student.

I give directions:

Students, fold your worksheet in half "hot dog" style (top to bottom). We will be working one side at a time. Solve your addition facts first, then unfold the worksheet. Now find the addition fact that can help you solve each subtraction fact and draw a line connecting them together. Now solve for the difference on the subtraction equation. 

You may note that the worksheet you print has the equations in a different order than my student's worksheets in the pictures. After they completed their work, I noticed the subtraction equations were not in a random order and my some of my kids picked up on a pattern for answering.  I fixed this issue for you.

Lesson Extension

5 minutes

I taught this lesson on doubles addition facts previously, and my students really embraced the mnemonics of the poster we created. This poster was created using Velcro, so I decided to leave it up to use in a center. This is excellent practice for students to use to reinforce the doubles facts. Here you can watch the boys practicing doubles video.