Have you used any of the numerical constraint solving libraries available for many Prolog implementations?[1] (It is also worth noting that Turbo Prolog is a rather peculiar—and now rather old—take on Prolog. I believe it has been superseded by Visual Prolog).

[1]: e.g., SWI Prolog's http://www.swi-prolog.org/pldoc/man?section=clpfd, or http://www.swi-prolog.org/pldoc/man?section=clpqr