2.NBT.8: Mentally Adding and Subtracting 10 or 100

I can mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100.

What Your Child Needs to Know

This standard focuses on helping your child develop mental math skills for quickly adding or subtracting 10 or 100 to three-digit numbers. Students will learn to recognize that when you add or subtract 10, only the tens digit changes, and when you add or subtract 100, only the hundreds digit changes.

This mental math skill builds on place value understanding and prepares your child for more efficient computation strategies in future grades. Mastering these skills will help your child develop number sense and computational fluency with larger numbers.

Real World Practice

Visual models and hands-on activities

Visual Models to Use

  • Hundreds Chart - A chart showing numbers from 100-1000 that helps visualize how adding or subtracting 10 moves down or up one row, while adding or subtracting 100 moves down or up one column.
  • Place Value Chart - A chart with columns for hundreds, tens, and ones that shows which digit changes when adding or subtracting 10 or 100.
  • Number Line - A line showing numbers in sequence with jumps of 10 or 100 marked to visualize these mental math operations.
  • Base-Ten Blocks - Physical or virtual blocks showing how adding or subtracting 10 means adding or removing one ten-rod, while adding or subtracting 100 means adding or removing one hundred-flat.

Everyday Activities

1. Elevator Math

When in a building with an elevator, ask your child to mentally calculate what floor you'll be on if you go up 10 floors or down 10 floors from your current floor.

2. Shopping Budget Game

Give your child a pretend budget of $500. Have them mentally add or subtract $10 or $100 as they "purchase" items or "earn" money.

3. License Plate Challenge

When you see a three-digit number on a license plate, have your child mentally add 10, add 100, subtract 10, and subtract 100 from it.

4. Calendar Countdown

Use a calendar to count forward or backward by 10 days or 100 days from today's date, discussing what the new date will be.

Quick Checks

Strategies and quick activities

Strategies When Your Child Struggles

1. Focus on Place Value

When your child struggles, remind them that adding 10 only changes the tens digit, and adding 100 only changes the hundreds digit. Use a place value chart to show this visually.

2. Use Counting Patterns

Practice counting by 10s and 100s from different starting points to help your child recognize the patterns (e.g., 347, 357, 367... or 347, 447, 547...).

3. Start with Easier Numbers

If your child struggles with numbers like 347, start with simpler numbers like 300 or 350 before moving to more complex numbers.

4. Connect to Real Life

Use money examples: adding a $10 bill or a $100 bill to a total, or taking away a $10 bill or a $100 bill from a total.

5-Minute Practice Activities

1. Mental Math Race

Call out a three-digit number (like 356) and ask your child to quickly add 10, then add 100, then subtract 10, then subtract 100. Time how fast they can do it.

2. Number Pattern Hunt

Write a three-digit number (like 472). Have your child write what happens when they add 10, add 100, subtract 10, and subtract 100, then explain the pattern they notice.

3. Digit Detective

Call out a calculation like "325 plus 10" or "648 minus 100" and have your child identify which digit will change and what the new number will be.

4. Quick Quiz

Create a set of 5 quick mental math problems (like 245 + 10, 789 - 100) for your child to solve without writing anything down.

Check Progress

Track improvement

Mid-Year Expectations

By the middle of second grade, your child should be able to:

  • Mentally add 10 or 100 to numbers between 100-500
  • Mentally subtract 10 or 100 from numbers between 100-500
  • Understand that adding/subtracting 10 changes only the tens digit
  • Understand that adding/subtracting 100 changes only the hundreds digit

End-of-Year Expectations

By the end of second grade, your child should be able to:

  • Fluently add 10 or 100 to any number between 100-900
  • Fluently subtract 10 or 100 from any number between 100-900
  • Explain the place value patterns when adding or subtracting 10 or 100
  • Apply these mental math skills in problem-solving situations

Mastery Signs

Your child has mastered this standard when they can:

  • Quickly and accurately add or subtract 10 or 100 mentally
  • Explain which digit changes and why when adding or subtracting 10 or 100
  • Apply these skills to solve real-world problems
  • Use these mental math skills as building blocks for more complex calculations
  • Identify and correct their own errors when they make them

Differentiation

Support for all learning levels

Below Grade Level

Practice problems focusing on adding and subtracting 10 to two-digit numbers, building the foundation for working with three-digit numbers.

Download Practice Worksheet

At Grade Level

Standard practice with mentally adding and subtracting 10 or 100 to numbers between 100-900.

Download Grade Level Worksheet

Above Grade Level

Challenging problems involving mentally adding and subtracting 10, 100, and 1000 to various numbers and applying these skills in multi-step word problems.

Download Challenge Worksheet