Addition Method

http://www.khanacademy.org/math/algebra/systems-of-eq-and-ineq/fast-systems-of-equations/e/systems_of_equations_with_elimination_0.5

These are Khan Academy practice problems that relate to the addition/elimination method.  These problems are one-step elimination problems.  It is recommended to watch all videos in this section prior to attempting this practice set.

http://www.khanacademy.org/math/algebra/systems-of-eq-and-ineq/fast-systems-of-equations/e/systems_of_equations_with_elimination

These Khan Academy practice problems relate to the addition/elimination method as well, but involve more "work" to eliminate one of the variables