Unit Plan 13 (Grade 2 Math): Mentally Add/Subtract 10s & 100s

Focus: Mentally add or subtract 10 or 100 from numbers 100–900 and justify using place value and structure.

Grade Level: 2

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

Total Unit Duration: 5 sessions (one week), 35–45 minutes per session

I. Introduction

Students build mental-flexibility with three-digit numbers by using place-value structure to add or subtract 10 or 100 without paper-and-pencil. They look for and make patterns (MP.7), use landmark numbers, and explain why the tens or hundreds place changes (and when it does not).

Essential Questions

• How does place value help me change a number by 10 or 100 in my head?
• What patterns do I notice when I add/subtract 10s or 100s?
• How can I explain why my mental method works?

II. Objectives and Standards

Learning Objectives — Students will be able to:

1. Mentally add or subtract 10 and 100 from any number 100–900 and state the new number.
2. Describe how the tens or hundreds digit changes (and when other digits stay the same).
3. Use place-value language to justify strategies and identify patterns (e.g., “only the tens digit changes”).
4. Choose efficient mental strategies (landmarks, compensation) and check reasonableness quickly.

Standards Alignment — CCSS Grade 2 (spiral across the unit)

• 2.NBT.8: Mentally add 10 or 100 to a given number 100–900, and mentally subtract 10 or 100 from a given number 100–900.
• MP.7: Look for and make use of structure (place-value patterns).

Success Criteria — Student Language

• I can add or subtract 10/100 in my head and say the answer quickly.
• I can explain which digit changes and why using hundreds/tens/ones language.
• I can point out a pattern (for example, “adding 10 changes the tens digit by +1”).
• I can do a quick reasonableness check using a hundreds chart or open number line.