This website explains and defines basic subtraction. There are practice problems at the very end of this lesson.

Practice rounding to the nearest ten and rounding to the nearest hundred. Students receive immediate feedback and have the opportunity to try questions repeatedly, watch a video tutorial, or receive hints about rounding.

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.