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

[Free Registration/Login may be required to access all resource tools.] The video describes how to convert between thousands and hundreds. The video is followed by practice problems to assess student understanding of the concept.

Use the Visualize It! tool to model word problems. This interactive exercise focuses on solving systems of equations by visualizing the problem or writing expressions then you get to make you own model and write your own word problem to...