Khan Academy
Khan Academy: Arithmetic and Pre Algebra: Adding, Subtracting Negative Numbers
This video teaches addition and subtraction of negative numbers using a number line. Practice problems are available. [9:16]
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Professor Freedman's Video Snippets Add/subtract Integer
Professor Ellen Freedman uses this very short video to explain how to add and subtract positive and negative integers. She offers tips on how to keep track of whether the answer is positive or negative.