Note that both numbers are multiple of 3: you can simplify the ratio as follows
[tex] \dfrac{6}{603} = \dfrac{3\cdot 2}{3\cdot 201} = \dfrac{2}{201} [/tex]
The ratio can't be further simplified because 2 and 201 are coprime. So, the only thing you could do is write down the decimal expansion of the number.
For the record, its period is 33 digits long, and it is
[tex] 0.009950248756218905472636815920398 [/tex]
