though and then it's it's good to know and good to take advantage of mathematically but you don't want to build your systems that way necessarily then X given Y is the probability of X taking on value of small X given Y has value small Y so what we have here is that as a definition we write it as X given Y is the probability of x and y divided by probability of Y so it think again about that rectangle there's a region where Y takes on the values small Y and that has a certain surface and that's py and we look at the region