Subtraction Strategies Unpacked

Grade Level: K-2

Duration: 90 minutes

Number of Students: 3

Unit: Mastering Math Concepts Together – Lesson 6 of 17

---

Common Core Standards Addressed

• Kindergarten:
  CCSS.MATH.CONTENT.K.OA.A.1 - Represent addition and subtraction with objects, fingers, mental images, drawings, sounds, acting out situations, verbal explanations, expressions, or equations.
  CCSS.MATH.CONTENT.K.CC.A.2 - Count forward beginning from a given number within the known sequence (instead of having to begin at 1).

• Grade 1:
  CCSS.MATH.CONTENT.1.OA.B.4 - Understand subtraction as an unknown-addend problem.
  CCSS.MATH.CONTENT.1.MD.B.3 - Tell and write time in hours and half-hours using analog and digital clocks (supports sequencing activities).

• Grade 2:
  CCSS.MATH.CONTENT.2.NBT.B.5 - Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.
  CCSS.MATH.CONTENT.2.MD.C.8 - Solve word problems involving dollar bills, quarters, dimes, nickels, and pennies.

---

Learning Objectives

By the end of this 90-minute session, students will be able to:

1. Use counting-back strategies to solve subtraction problems fluently within 20 (K-1), or within 100 (Grade 2).
2. Use number lines to visualize and solve subtraction problems.
3. Collaboratively create and solve original subtraction problems using manipulatives or drawings.
4. Explain their subtraction strategies verbally and through drawings to peers.

---

Materials Needed

• Individual number lines (paper strips marked 0–20 for K-1; 0–100 for Grade 2)
• Small counters or cubes (20 per student for K-1; up to 100 for Grade 2)
• Whiteboards and markers
• Subtraction problem cards (starting problems and blanks for creation)
• Chart paper and markers for group brainstorming
• Timer or stopwatch
• Sticky notes for reflection
• Visual aids: Illustrations of subtraction (objects being taken away, number lines, and empty boxes)

---

Lesson Outline

1. Warm-Up & Engage (10 minutes)

• Activity: "Subtraction Story Time" — Teacher narrates a simple subtraction story using objects (e.g., “Five apples on the table, 2 eaten, how many left?”).
• Student Action: Students use counters to model the story and count back to solve.
• Purpose: Engage with hands-on demonstration aligned with CCSS.K.OA.A.1 and CCSS.1.OA.B.4.

2. Direct Instruction: Strategies Overview (15 minutes)

• Introduce two subtraction strategies:
  • Counting Back: Demonstrate counting backward on fingers and with counters.
  • Using a Number Line: Introduce number line visualization, showing jumps backward.
• Use visual aids and interactive mini whiteboards.
• Link back to standards: Counting back aligns with CCSS.K.CC.A.2 and CCSS.2.NBT.B.5.

3. Guided Practice: Hands-On Collaborative Exploration (25 minutes)

• Activity Setup:
  • Students sit together and receive number lines and counters.
  • Each student creates 1 subtraction problem using counters (e.g., “I have 12 cubes, I take away 4, how many left?”).
  • The other two solve the problem using counting-back and number lines.
• Teacher Role: Circulate, facilitate discussion on methods used, encourage verbal explanations of their thinking process.
• Expected Outcome: Build conceptual understanding, peer teaching, support CCSS.2.NBT.B.5 by exploring multiple strategies.

4. Independent Collaborative Challenge (20 minutes)

• Task:
  • Using whiteboards and markers, students collaboratively create a mini "Subtraction Storybook" with 3 problems reflecting real-life contexts (apples, toys, etc.).
  • Each problem must be solved using both strategies: counting back and number line.
• Focus: Emphasizes verbal and visual explanation, creativity, collaboration skills, and application of learned concepts. Supports CCSS.K.OA.A.1 and CCSS.1.OA.B.4.

5. Group Reflection and Sharing (15 minutes)

• Each student shares one created subtraction problem and explains their preferred method.
• Teacher charts main strategies and insights from student problem-solving.
• Encourage students to use mathematical vocabulary such as “subtract,” “count back,” “number line,” “difference,” and “take away.”

6. Formative Assessment & Closure (5 minutes)

• Exit Ticket on Sticky Notes: Write one new thing learned about subtraction strategies.
• Teacher quickly reviews and gives verbal feedback.

---

Assessment and Evaluation

Formative

• Observation of students’ use of manipulatives and strategies during guided and independent tasks.
• Anecdotal notes on verbal explanations and collaboration skills.

Summative (At Unit End)

• Review of the Subtraction Storybook problems for correctness and strategy explanation.
• Use of rubrics aligned with CCSS mathematical practices for explanation and accuracy.

---

Differentiation & Supports

• For Struggling Students: Use more physical manipulatives and one-on-one guided counting back with teacher prompts.
• For Advanced Students: Challenge them to create subtraction problems within 100 or introduce missing-addend strategies (1.OA.B.4).
• English Language Learners: Use visuals and gestures, sentence frames for explaining strategies ("I subtracted ___ by counting back ___ steps").

---

Teacher Reflection Notes

• How effectively did students articulate their subtraction thinking?
• Were both selected strategies accessible and engaging?
• What scaffolding was most beneficial for each student?
• Adjust upcoming lessons to reinforce student-developed areas of challenge, e.g., number lines or verbal strategy explanations.

---

This highly interactive lesson allows students to embody the mathematical practices of reasoning abstractly and quantitatively while honing concrete subtraction skills aligned to their developmental stage and Common Core Standards. It also nurtures collaboration — a core 21st-century skill.