solve open sentences involving addition and subtraction.

• http://www.beaconlearningcenter.com/WebLessons/Circus/default.htm

• http://www.beaconlearningcenter.com/WebLessons/ItsAFact/default.htm#page1

• http://www.oswego.org/ocsd-web/games/SumSense/sumadd.html

• http://www.oswego.org/ocsd-web/games/SumSense/sumsub.html