Math Background

Lesson: Addition and Subtraction Facts to 20
Introducing the Concept

Begin by focusing on addition facts. When children have become proficient with these, introduce the corresponding subtraction facts. Set the facts in the context of a story. Act them out and encourage children to join in the activity. If manipulatives are available, you may want to repeat the activities with different numbers while children use manipulatives to act them out. Once children are comfortable with all the facts, encourage them to use the fact cards for practice.

Materials: fact cards; primary and regular crayons, 10 each

Preparation: Prepare fact cards for addition facts through 20 and for the corresponding subtraction facts. Have 10 large and 10 small crayons available for demonstration.

Prerequisite Skills and Concepts: Children should have a good foundation in number relationships.

  • Say: I need volunteers to help me act out an addition story.
    Choose 9 children to come to the front of the class. Have the rest of the class count the 9 children. Then ask 3 more children to come to the front of the room, and count again.
  • Ask: What did we do?
    Children should say that 3 more children joined the group, or 3 more children were added to the group.
  • Ask: What numbers should we use to show our story?
    (9 and 3)
    Write the addition sentence on the chalkboard.
    (9 + 3 = 12)
  • Say: Let's try it again.
    Continue as above, using 9 children in the original group with 1 more child joining the group each time.
  • Ask: Do you see a pattern here?
    Children should see that the number added and the sum went up by 1 each time.
  • Say: Now we are going to try something different.
    Repeat the activity, only this time call a different number of children to represent one addend.
    Have children count the number.
  • Say: You will ask zero more children to join them.
    Pause for children to consider the situation.
  • Ask: What did you notice this time?
    Children should see that if no children are added to the group, the number remains the same (for example, 8 + 0 = 8, 10 + 0 = 10).
    Reverse the procedure and have zero children go to their seats.
  • Say: If I have 10 children here and I don't tell anyone to go to his or her seat, how many are left?
    Ten are left. On the chalkboard, write 10 − 0 = 10. Help children to see that when zero is subtracted from a number, the sum is that number.
  • Say: I would like 6 boys to line up at the front of the classroom. Now I need 4 girls to line up beside the boys.
  • Ask: What does this show you?
    There are 6 boys and 4 girls; 6 and 4 more are 10. On the chalkboard, write 6 + 4 = 10, or place the fact card in the chalk tray. Repeat the activity, but ask the girls to line up first.
  • Say: Now let's have 4 girls line up first, then 6 boys. What do you see?
    There are 4 girls and 6 boys; 4 and 6 more are 10. On the chalkboard, write 4 + 6 = 10, or place the matching fact card in the chalk tray.
  • Ask: What did we do that was the same? What did we do that was different?
    The same number of children were lined up both times, but the order of the boys and girls changed. Make sure children recognize that the order of the addends didn't change the sum. Repeat with some other facts.
  • Say: If you know one addition fact, you can change the order of the addends and you will know the related fact. So if you know one fact, you know two facts.
  • Say: Look at what I have on the table. I have placed 8 big crayons here. Now I am going to add 5 little crayons.
  • Ask: What number sentence can you show for the crayons?
    8 + 5 = 13. Write it on the chalkboard, or show the fact card.
  • Ask: How can I take apart what we just did?
    Take away the 5 little crayons.
  • Ask: What number sentence shows what I just did?
    13 − 5 = 8. Write the number sentence on the chalkboard. Tell children that these two facts are related. If they know an addition fact, they know a subtraction fact too!
  • Ask: What other addition fact can you make with 8 + 5 = 13?
    Elicit the answer 5 + 8 = 13. Write it next to 8 + 5 = 13. Demonstrate the new fact with the crayons. Discuss how the two number sentences are the same and how they are different.
  • Ask: What other subtraction fact can you make with 13, 8, and 5?
    Children should say that they can make 13 − 8 = 5. Demonstrate the new subtraction fact with the crayons.
  • Say: These four facts make a fact family. If you know one of these facts, you can figure out all the others in the family! Now, how can we make a fact family with 7, 6, and 13?
    Help students write the four sentences in the family on the chalkboard.
  • Ask: Let's try another one. I need 7 girls to come to the front of the class. Now, I need 7 boys to join them. What fact can you write?
    The fact is 7 + 7 = 14. Write the fact on the chalkboard.
  • Ask: What happens if the 7 boys go back to their seats? What is the fact?
    Children should say that there are 7 girls left. 14 − 7 = 7. Write that fact on the chalkboard. Have children look at both facts and discuss what they see.
  • Ask: Can you write any more facts for this family?
    Have the same seven boys and seven girls line up in reverse order. Encourage children to see that if they reversed the order of doubles such as 7 + 7, the facts would be the same. Help children conclude that doubles have only two members to their family—one addition fact and one subtraction fact.

Houghton Mifflin Math Grade 2