Well, this seems pretty straightforward. The first term is the non-repeating portion of the fraction and the 2nd term is the repeating portion, where the number of 9's is the period of repetition.

For instance, 1/7 = .142857142857142857 ... = 142857/999999

53/35 = 1.5142857142857142857 ...

= 15/10 + 142857/10*999999

The representation is not unique.

1/3 = .333333.. = 3/9 = 3/10+ 3/90 = 33/100 + 3/900, etc.

1/7 = 142857/999999

= 1/10 + 428571/9999990

= 14/100 + 285714285714/99999999999900

I think it possible to require C = D, and still have every positive fraction expressible in that form.