http://www.singularsys.com/autoabacus

AutoAbacus is a powerful equation solving library that finds solutions
to equation sets with a snap. Equations are passed to AutoAbacus as
text, while the program attempts to find a solution that satisfies all
constraints.

The equations are not limited to be only linear, but can also be
polynomial or include arbitrary functions. By profiling the types of
equations in the system and their dependencies on each other,
AutoAbacus uses appropriate solution methods to solve individual
subsets of equations.

Applications range from use in a business rules engine to solving
engineering equation systems.