[microcolonel]

Rational numbers are a good way to do many financial calculations (extra points for doing something useful with negative denominators!), since many financial calculations are specified with particular denominators (per cent, per mille, basis points; halves, sixths, twelfths, twenty-sixths, fifty-seconds of a basis point, etc.).

However, as soon as you start doing anything interesting, you have limited precision as a matter of course.