Would forcing the integer result fix this problem? Something like:

    def f(a, b, c, d, e, f, g, h, i):
        return a + int((13 * b * i + g * h * c) / (i * c)) + d + 12 * e - f - 11 - 10

If I try it like this, I still have the same number of solutions... Maybe I'm missing something..

EDIT: code formatting...