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Third/Addition

Addition: Complete One Hundred ()

Find the missing number that makes 100. Practice mental math strategies to reach 100. Example: ___ + 85 = 100 (Answer: 15)
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Quick Tip
To find what adds to 100, subtract the given number from 100.
Teacher Resources
Teaching Notes

This activity strengthens mental math by finding complements to 100. Encourage students to use various strategies like counting on or breaking apart numbers.

Vocabulary
Target number: The number you aim to reach.
Difference: The result of subtraction.
Common Mistakes
  • Forgetting place value when adding (e.g., 90 + 15 instead of 90 + 10).
  • Basic calculation errors, especially with numbers that require regrouping.
  • Not leveraging known facts (e.g., complements to 10) to build to 100.
Differentiation
SupportUse a number line, base-ten blocks, or ten-frames for visualization.
ChallengeExtend to finding complements to 1000 or using three addends.
Discussion Questions
  • What strategy did you find most helpful for completing 100?
  • How can knowing sums to 10 help you quickly make 100?
  • Can you think of a real-life situation where you might use this skill?
  • If you know 25 + 75 = 100, what other related facts can you find?
Extension Activities
  • Reach 100 using different strategies.
  • Find pairs that add to 100.
Parent Tip

Count pennies to make a dollar (100 cents).

Learning Path
Skill Cluster

Number Sense & Mental Math

Estimated Time

15 minutes

Skills Practiced
addition to 100subtraction within 100
Prerequisites
  • Addition within 20
  • Complements to 10
  • Place Value to 100
Next Steps
  • Addition to 1000 (missing addend)
  • Subtraction from 100
  • Mental Math with larger numbers
  • Addition: Complete One Thousand
  • Subtraction: Find the Difference from 100
  • Mental Math: Making Tens and Hundreds