Other
Quick Math: Algebra Simplify
This QuickMath algebra site simplifies an advanced expression that you enter and gives answer.
National Council of Teachers of Mathematics
The Math Forum: Credit Card Calculations
At this site Dr. Math answers a question about credit card balance payoffs. An advanced algebraic solution is offered.
Wyzant
Wyzant: Worksheet Index
WyzAnt is a resource of many algebra worksheets. An answer key is available, or you can click on the problem to see the work. Topics include: basic algebra, simplifying, multiplication, exponents, order of operations, negative exponents,...
Cut the Knot
What's a Rational Number?
This question is answered by discussing and comparing rational and irrational numbers. For more advanced levels of Algebra, there are sections that discuss different lemmas of rational numbers and give their proofs.