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Grade 1 Math Units

Unit Plan 10 (Grade 1 Math): Subtraction within 20—Decomposing

Develop Grade 1 subtraction within 20 by using PPW, ten frames, and number lines to count back or count on, bridge to 10, and match models to precise equations.

Dr. Michael Kester-Haynes

• 5 min read
Unit Plan 10 (Grade 1 Math): Subtraction within 20—Decomposing

Focus: Use take-away, part–part–whole (PPW), and number line models to develop subtraction strategies: count back, count on (unknown addend), decompose to make/bridge 10, and record precise equations.

Grade Level: 1

Subject Area: Mathematics (Operations & Algebraic Thinking)

Total Unit Duration: 5 sessions (one week), 45–60 minutes per session

I. Introduction

Students strengthen subtraction within 20 by connecting stories to models (counters, ten frames, number lines, number bonds). They compare strategies—take-away, count back, and count on as unknown addend—and explain why each works. Emphasis is on flexible thinking, tool choice, and clear equations.

Essential Questions

  • How do models (PPW, ten frames, number lines) help me subtract within 20?
  • When is it more efficient to count on (unknown addend) instead of take away?
  • How can I decompose numbers (bridge to 10) to make subtraction easier?

II. Objectives and Standards

Learning Objectives — Students will be able to:

  1. Represent subtraction within 20 using take-away, PPW/number bonds, and number line strategies.
  2. Use count back and count on (solve as unknown addend) to find differences efficiently.
  3. Decompose numbers to bridge 10 (e.g., 13 − 6 → 13 − 3 − 3 → 10 − 3).
  4. Choose and justify a tool and equation that match the situation; check for reasonableness.

Standards Alignment — CCSS Grade 1 (threaded across the unit)

  • 1.OA.4: Understand subtraction as an unknown-addend problem.
  • 1.OA.5: Relate counting to subtraction (e.g., count back/count on).
  • 1.OA.6: Add and subtract within 20 using strategies such as decomposing, making/bridging 10, and using the relationship between addition and subtraction.
  • Mathematical Practices: MP.5 (Use appropriate tools strategically) emphasized; MP.2, MP.6 threaded.

Success Criteria — Student Language

  • I can show subtraction with PPW, a number line, or ten frames and write a matching equation.
  • I can count on to find a missing part (unknown addend) or count back to take away.
  • I can decompose to make/bridge 10 and explain why my strategy is efficient.