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.
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.