Solving Systems of Equations
This site gives a straight-forward example solving a linear equation.
http://regentsprep.org/Regents/math/syslin/GrSys.htm

Supply and Demand
This site gives a nice introduction to supply and demand.
http://www.cssd.ab.ca/tech/social/tut9/lesson_13.htm

Solving Linear Inequalities
This site discusses solving linear inequalities and it provides some examples.
http://home.xnet.com/~fidler/triton/math/review/mat085/linineqtwo/linineqtwo1.htm