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Addition - Math is Fun
Addition worksheets for K-5 - K5 Learning
Basic addition - Math made easy - YouTube
Basic Addition Facts - Super Teacher Worksheets
Free Addition Worksheets—Printable with Answers - Mashup Math
Basic Addition & Subtraction Facts — Online Practice (grades 1-2)
Unit 2: Addition and subtraction - Khan Academy
Khan Academy
Addition in Math: Definition, Examples, Formulas
Addition - Math.net