Teach students how to add and subtract both negative and positive integers with this system that helps students visualize operations by relating them to the filling or draining of a toilet tank. The system can be used to teach younger students basic addition and subtraction, then reintroduced to older students learning how to add and subtract equations involving negative integers. The 195-page activity book explains the system, lists the covered NCTM Standards, and provides worksheets to accommodate all levels and abilities. The double-sided dryerase boards feature 2 different Watertank diagrams, 1 for beginners and 1 for more advanced students. Kit includes an activity book and a CD-ROM with PDF versions of the activities.