Standard

Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900.

Student language:

• "I can add and subtract tens and hundreds in my head."

Explanation

About the Math, Learning Targets, and Rigor

This standards calls for students to mentally add and subtract multiples of 10 or 100 to any number between 100 and 900. Students should have ample experiences developing proficiency with mental computation.

Mentally adding and subtracting 10 or 100 to a given number understanding that they are only changing the tens place (multiples of ten) or the digit in the hundreds place (multiples of 100).

Working with place value blocks and a hundreds chart before moving to mental computation will be beneficial for students. Be sure to include numbers do not just end in 5 or 0 (like 10, 55, 80, etc.).

Examples

• 100 more than 653 is (753)
• 10 less than 87 is (77)
• “Start at 248. Count up by 10s until I tell you to stop.”

Common Misconceptions

• Students may think that the 4 in 46 represents 4, not 40 or 4 tens. Students need many experiences representing two and three-digit numbers with manipulatives that group (base ten blocks) and those that do NOT group, such as counters, etc.
• When adding two-digit numbers, some students might start with the digits in the ones place and record the entire sum. Then they add the digits in the tens place and record this sum. Assess students’ understanding of a ten and provide more experiences with manipulatives like base-ten blocks.
• When subtracting two-digit numbers, students might start with the digits in the ones place and subtract the smaller digit from the greater digit. Then they move to the tens and the hundreds places and subtract the smaller digits from the greater digits. Assess students’ understanding of a ten and provide more experiences with manipulatives like base-ten blocks.

Resources

Videos

• TEACHING VIDEO - coming soon
• Khan Academy
  • scroll to find 2.NBT.8, click on a skill, watch videos on left side in each skill
• Learn Zillion

EngageNY Lessons

Extra Practice

• North Carolina Tasks
• Illustrative Math Tasks
• Howard County Tasks
  • Click on 2nd grade at bottom, then 2.NBT.8
• Common Core Sheets

Released Assessment Questions

2.NBT.8 Student Example

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