Assuming it's the "580" block being repeated:
x = 0.580580580...
1000x = 580.580580580...
1000x - x = 999x = 580
x = 580/999
If it's the "80" block that repeats:
x = 0.5808080...
10x = 5.808080...
1000x = 580.808080...
1000x - 10x = 990x = 575
x = 575/990 = 115/198
And if it's just the 0 that repeats, then the decimal terminates and
0.58000... = 0.58 = 58/100 = 29/50
but this is likely not the number you're talking about.
