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Discovery Education
Discovery Education: Web Math: How to Round a Number
This site provides a description and helpful hints on how to round numbers to any place value. Content also includes an interactive problem solver complete with a full description of how the answer was accomplished.
Lawrence Hall of Science
The Math Page: Skill in Arithmetic: The Meaning of Subtraction, and Mental Calculation
This website explains and defines basic subtraction. There are practice problems at the very end of this lesson.