Lesson 1

Strategies for Subtraction Facts

Est. Class Sessions: 2–3

Developing the Lesson

Part 1: Revisiting the Subtraction Strategies Menu for the Facts

Subtraction Strategies for the Facts. Remind students that earlier in the year they sorted subtraction facts into two categories: Counting Strategies and Reasoning Strategies. For today’s lesson, students will use the Subtraction Strategies Menu for the Facts in the Student Activity Book Reference section to sort subtraction facts into six categories: counting up, counting back, making ten, using ten, using doubles, and thinking addition. Use a display of the Subtraction Strategies for the Facts page to review each of the subtraction strategies.

Display the 6 sheets of chart paper and the Show Your Strategy Cards with the facts 12 − 10, 10 − 4, 7 − 2, 7 − 5, 14 − 7, and
15 − 7. See Materials Preparation. Focus students’ attention on the displayed cards and ask students to explain how they would use each of the strategies to solve the displayed problems. As students give their explanations, post the subtraction fact on the chart that matches the strategy.

Ask:

X
  • Look at the subtraction facts on the six strategy cards. How can you use the using-ten strategy to solve 12 − 10? (Possible response: I know 10 + 2 = 12 so 12 − 10 = 2.)
  • How would you use the making ten strategy to solve 10 − 4? (Possible response: I know that 6 + 4 = 10 so 10 − 4 = 6.)
  • How would you use the counting back strategy to solve 7 − 2? (Possible response: I would put 7 in my head and count back 2: 6, 5.)
  • How would you use the counting up strategy to solve 7 − 5? (Possible response: I would put 5 in my head and count up to 7: 6, 7.)
  • How would you use the using doubles strategy to solve 14 − 7? (Possible response: I know that
    7 + 7 = 14 so 14 − 7 = 7.)
  • How would you use the thinking addition strategy to solve 15 − 7? (Possible response: I know that
    7 + 8 = 15 so 15 − 7 = 8.)
  • What are the four number sentences that belong to the same fact family as 15 − 7? (7 + 8 = 15,
    8 + 7 = 15, 15 − 7 = 8, 15 − 8 = 7)

Demonstrate how to write the four number sentences in the same fact family as 15 − 7 on a display. See Figure 3.

Explain to students that there may be more than one strategy for solving each of the problems but the goal is to use a strategy that will help them solve the problem quickly and accurately. For example, to solve 15 − 7, students could use the strategies counting back, counting up, using doubles, or thinking addition. Explain that counting back is a good strategy to use if they are subtracting small numbers and counting up is a good strategy to use if they are subtracting two numbers that are close to each other.

Sort Subtraction Facts. Have students work in pairs to discuss strategies for the remaining subtraction facts you prepared on the Show Your Strategy Cards Master. Give each pair 4–5 cards. Encourage students to refer to the Subtraction Strategies Menu for the Facts in the Student Activity Book Reference section before they solve each problem. If some pairs finish early, give them additional facts to solve. Ask each pair to solve the subtraction facts and show their strategy on the number line and ten frames on the strategy cards. As pairs complete their cards, have them attach the cards to the chart that corresponds to the strategy that they think is most efficient for solving the problem.

As students work on the strategy cards, ask questions such as the following:

X
  • What strategy did you use to solve the problem?
  • Is there another strategy you could have used to solve this problem?
  • Is the strategy you used an efficient strategy for solving the problem?
X

Students in Second Grade will rely heavily on counting strategies to solve subtraction problems. Encourage students to use other strategies by asking which strategy is most effective or efficient for a particular subtraction fact. Counting strategies can be used to count up or back small numbers, such as 1, 2, or 3, but are inefficient when counting larger numbers, such as 7, 8, or 9. Ask students to try using a different strategy for problems that are not solved efficiently by using a counting strategy. Students will need ongoing practice with the strategies on the Subtraction Strategies Menu for the Facts before they move beyond using counting strategies to solve all subtraction problems.

Discuss Subtraction Strategies Charts. Upon completion, direct students’ attention to the strategy cards on the six charts. Ask students to look at the facts on each chart and write down subtraction facts they would like to move to another chart. As a class, discuss the placement of the subtraction facts on the six charts.

Ask:

X
  • Are there some facts that you would like to move from one chart to another? Why?
  • Are there some facts that can be solved using more than one strategy? Give an example.
  • Are there some facts on the “Counting Up” or “Counting Back” charts that you would move to another more efficient strategy?

Have students move strategy cards to a different chart if they find more efficient strategies for solving the problems. Ask students to explain how they would use the strategy to solve the problem. Remind students that they should use strategies that are efficient and make sense to them and that some subtraction facts can be solved efficiently using more than one strategy.

ADVENTURE BOOK:
Student Activity Book:
Master:
Fact family and strategy for solving 15 − 7 = 8
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Subtraction Strategies Menu for the Facts
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