0.125555(repeating)
First, split the non-repeating from the repeating
0.12 + 0.005555(repeating)
Next, move the decimal place to offset the leading zeros. Divide by the appropriate multiple of 10 (if one zero, it's over 10; if two zeros, over 100).
0.12 + 0.555(repeating) / 100
Any single digit repeating decimal is represented as that number over 9. (side note: if it were two digits repeating, it would be over 99, and if three, 999.. and so forth)
0.12 + (5/9) / 100
And 0.12 is the same as 12/100
12/100 + (5/9)/100
To solve that complex fraction (the second fraction), multiply top and bottom by 9 (which is the same as multiplying by 9/9, or 1, and multiplying by 1 does not change the expression).
12/100 + 5/(9*100)
12/100 + 5/900
Solve the fraction as normal.
108/900 + 5/900
113/900
If you can plug the fraction 113/900 into your calculator, it will validate the decimal representation.