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.