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Grade 1 Math Units

Unit Plan 19 (Grade 1 Math): Add/Sub within 100—Expanded Form

Add and subtract two-digit numbers within 100 using expanded form, base-ten models, and open number lines, while explaining each step with clear place-value reasoning.

Dr. Michael Kester-Haynes

• 6 min read
Unit Plan 19 (Grade 1 Math): Add/Sub within 100—Expanded Form

Focus: Solve two-digit problems using models, expanded form, and open number lines; explain choices and reasoning with place-value language.

Grade Level: 1

Subject Area: Mathematics (Number & Operations in Base Ten)

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

I. Introduction

Students connect tens and ones to computation by writing numbers in expanded form, building with base-ten blocks, and using open number lines. They add and subtract within 100 (including multiples of ten) and justify why their steps make sense.

Essential Questions

  • How does expanded form help me add or subtract two-digit numbers?
  • When is an open number line or base-ten model the best choice?
  • How do tens and ones guide my steps so my answer is reasonable?

II. Objectives and Standards

Learning Objectives — Students will be able to:

  1. Represent two-digit numbers with tens and ones, and write them in expanded form.
  2. Add within 100 using expanded form, base-ten models, and open number lines (two-digit + one-digit; two-digit + multiple of ten).
  3. Subtract multiples of ten from two-digit numbers using models and number lines.
  4. Explain and check reasonableness by attending to tens and ones.
  5. Choose a representation and use math words to describe each step.

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

  • 1.NBT.4: Add within 100 using models/strategies based on place value and the relationship between addition and subtraction.
  • 1.NBT.5: Mentally find 10 more/10 less than a given number and explain reasoning.
  • 1.NBT.6: Subtract multiples of ten (10–90) from multiples of ten within 100 using models/strategies.
  • Mathematical Practices: MP.2 emphasized (reason quantitatively); MP.1, MP.4, MP.6 threaded.

Success Criteria — Student Language

  • I can write a number in expanded form (e.g., 58 = 50 + 8) and show it with tens and ones.
  • I can add or subtract using an open number line or base-ten blocks and explain each jump or trade.
  • I can check if my answer makes sense by using tens/ones and a quick estimate.