All probabilities can be represented not just as a ratio (like 7/19) but as a decimal between 0 and 1. We can do this simply by dividing. (and of course that makes it between 0 and 1 because you can't have the numerator be larger just like you can't have more success than total chances) 0 = no chance, 1 = 100% chance, and then there's everything inbetween...
