Strike it Out

Resources and Tools for Elementary Math Specialists and Teachers

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This game for two players provides students with practice in whole number addition and subtraction while developing strategic thinking. It is played on paper on a 0-20 number line. Player 1 crosses off two numbers and circles either their sum or difference. In each successive turn players cross off the last circled number and one of their choice, and then circle the sum or difference. The player who makes the last possible move wins. The game also may be played co-operatively. The Teachers' Notes page offers suggestions for introducing the game, discussion questions, and extension ideas.

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Contributed by: Millennium Mathematics Project, Publisher

Math TopicNumber Sense, Basic Operations, Number Concepts, Mathematical Practices, Mathematical Processes

Grade Level1, 2, 3, 4, 5

Resource TypeActivity, Game

Additional Information
- AudienceLearner
- LanguageEnglish (United Kingdom), Chinese, Dutch, French, German, Italian, Spanish
- Education Topic
- Interdisciplinary Connection
- Professional DevelopmentNo
- ContributorMillennium Mathematics Project, Publisher
- Publication Date2009-07
- RightsCopyright (c) 2007 University of Cambridge
  http://nrich.maths.org/public/terms.php
- AccessFree access
Standards
- Common Core State Standards for Mathematics
  Select a standards document:
- Standards for Mathematical Practice[K-12]
- Make sense of problems and persevere in solving them.[K-12]
- Reason abstractly and quantitatively.[K-12]
- Construct viable arguments and critique the reasoning of others.[K-12]
- Operations and Algebraic Thinking[K - 5]
- Understand and apply properties of operations and the relationship between addition and subtraction.[1]
- 4. Understand subtraction as an unknown-addend problem. For example, subtract 10 — 8 by finding the number that makes 10 when added to 8.[1]
- Add and subtract within 20.[1 - 2]
- 6. Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., 8 + 6 = 8 + 2 + 4 = 10 + 4 = 14); decomposing a number leading to a ten (e.g., 13 — 4 = 13 — 3 — 1 = 10 — 1 = 9); using the relationship between addition and subtraction (e.g., knowing that 8 + 4 = 12, one knows 12 — 8 = 4); and creating equivalent but easier or known sums (e.g., adding 6 + 7 by creating the known equivalent 6 + 6 + 1 = 12 + 1 = 13).[1]
- 2. Fluently add and subtract within 20 using mental strategies. By end of Grade 2, know from memory all sums of two one-digit numbers.[2]
- Work with addition and subtraction equations.[1]
- 8. Determine the unknown whole number in an addition or subtraction equation relating three whole numbers. For example, determine the unknown number that makes the equation true in each of the equations 8 + ? = 11, 5 = — 3, 6 + 6 = .[1]

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Strike it Out

Common Core State Standards for Mathematics