Math help.... Ration and Proportions [LTF]?

Sometimes you just have to start trying things and see what works. You have to know two things: what you are allowed to do, and whether it worked. In this case, you know that you can divide two numbers if the denominator is not zero, and you know it worked because (AB/AD)/(AC/AD) = (AB/AD)*(AD/AC) = AB/AC when you cancel ADs.