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# Subtraction Strategies

This lesson can be used as a pre-lesson for the Show Me the Money! Two-Digit Subtraction lesson plan.

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This lesson can be used as a pre-lesson for the Show Me the Money! Two-Digit Subtraction lesson plan.

Students will be able to show different ways to subtract multiples of 10.

##### Language

Students will be able to make sense of subtraction story problems and describe steps to solve them using visuals, manipulatives, and partner support.

(5 minutes)
• Ask students to give you a thumbs up if they have ever bought a stamp, and comment that your friend Susana loves to collect stamps from all over the world. Show students examples of different stamps. Build background knowledge about how stamps are used to send mail, but can also be collected.
• Tell students the following story problem: "Susana got \$30 from her grandma for her birthday. Susana spends \$20 on stamps for her collection. How much money does Susana have left?"
• Prompt students with questions about the context of the problem. For example, "How much money does Susana get for her birthday?" and "What words in the problem tell us which math operation we need to solve the problem?" Write what students know about the problem on the board.
• Ask students to retell the problem in their own words to a partner.
(5 minutes)
• Ask students to share ways to solve the problem. Students should say that they need to subtract, and then share a subtraction strategy.
• Record possible strategies on the board. Introduce different strategies as students suggest them or after students have stopped volunteering ideas.
• Draw a picture to model the problem. Draw three rectangles with \$10 inside each one. Cross off two of the rectangles.
• Sketch a horizontal line with tick marks of multiples of 10 from 10 to 30. Start at 30, and model drawing two jumps backwards to 10.
• Show students how to use the hundreds chart to count backwards to 10.
• Ask students how knowing that 3 - 2 = 1 helps them solve 30 - 20. Review that there are three tens in 30, and two tens in 20. Three tens minus two tens equals one 10. The numbers in the two equations are the same, but the units are different.
• Model writing a horizontal equation to solve the problem. Tell students that this type of equation is written from left to right. Have students gesture with arm in a horizontal position.
• Repeat the procedure with a vertical equation. Point out the the equal sign is written differently, but both equations show the same problem.
(10 minutes)
• Distribute the Stamps Subtraction worksheet to each student. Read the first problem together with the students: "Susana got 50 stamps for her birthday. Susana put 20 stamps away in her stamp book. How many more stamps still need to be put away?"
• Tell students to turn and talk to a partner about what is known about the problem, not how to solve it. Record what is known about the problem on the board.
• Next, ask students to turn and talk to a partner about how they would solve the problem. Choose student volunteers to suggest specific strategies (e.g., sketching a number line or writing an equation).
• Direct students to solve the problem independently, using one of the suggested strategies. Encourage students to show their work in a way that would help another student understand how they solved the problem.
(10 minutes)
• Direct students to solve the other two problems in small groups.
• As students work, circulate and prompt students to restate the problem in their own words.
• Prompt students to verbalize their thinking and show subtraction strategies with drawings and equations. Provide the sentence frame, "I solved the problem by ____."
• As students finish, ask if they can show the solution using a different subtraction strategy.

Beginning

• Read and solve the problems together in a teacher-led small group.
• Translate problems to the home language (L1), or partner student with a bilingual peer if possible.

• Ask students to write subtraction word problems for classmates to solve.
• Prompt students to restate problems and strategies for solving them in their own words.
(5 minutes)
• Circulate as students work on the problems. Notice whether students are using the same strategy for each problem, or using different strategies to solve different problems.
• Check that students are counting by tens rather than ones to solve the problems. Reteach subtracting multiples of 10 to students who count by ones to solve the problems.
• Choose student volunteers to restate the situation in problem #3 (how many more stamps Susana has than Greg) in their own words. Students may try adding to solve the problem because of the word "more." Note any misunderstandings, and address them during the closing section.
(5 minutes)
• Prompt students to describe the strategies that they used to solve the problems using the sentence frame, "I solved the problem by ____."
• Ask students how skip counting could be used to find the answer more quickly than counting by ones.

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