What I want to do in this video is build up some tools in our toolkit for dealing with sums and differences of random variables. So let's say that we have two random variables, X and Y, and they are completely independent. They are independent random variables. And I'm just going to go over a little bit of notation here. If we wanted to know the expected, or if we talked about the expected value of this random variable X, that is the same thing as the mean value of this random variable X. If we talk about the expected value of Y, that is the same thing as the mean of Y. If we talk about the variance of the random variable X, that is the same thing as the expected value of the squared distances between our random variable X and its mean.