The maximum decimal you can have is 1. 1.0, 1.00, 1.000, same thing.
To convert the decimal .123 into a fraction, we will have to put .123 as the numerator and the maximum decimal (which is 1) as the denominator.
0.123/1
Try to avoid decimals in fractions. In this case, multiply both the numerator and denominator by 1,000 to get rid of the decimal sign.
(0.123 * 1,000)/(1 * 1,000) = 123/1000
And we can't simplify it any further.