So the expected value of X, which I could also denote as the mean of our random variable X, let's say I expect to see three dogs a day, and similarly for the cats, the expected value of Y is equal to, I could also denote that as the mean of Y, is going to be equal to, and this is just for the sake of argument, let's say I expect to see four cats a day, and in previous videos, we defined how do you take the mean of a random variable, or the expected value of a random variable. What we're going to think about now is, what would be the expected value of X plus Y? Or another way of saying that, the mean of the sum of these two random variables. Well, it turns out, and I'm not proving it just yet, that the mean of the sum of random variables is equal to the sum of the means. So this is going to be equal to the mean of random variable X plus the mean of random variable Y. And so in this particular case, if I were to say, well what's the expected number of dogs and cats that I would see in a given day? Well, I would add these two means.