A ratio betewen two quanttiiesA and B is their fratcionA/B. A ratio is a mathemaitcal way of expressing the prooprtion of A to B. For exapmle, if I buoght 20 apples and have only 9 left, what is the prpoortion (or ratio) of 9 to 20? The anwser is 9/20 or 0.45/1. The ratio may also be writetn as 0.45.