The random variable X plus the random variable Y. What is the expected value of Z going to be? The expected value of Z is going to be equal to the expected value of X plus Y. And this is a property of expected values. I'm not going to prove it rigorously right here, but it's the expected value of X plus the expected value of Y. Or another way to think about this is that the mean of Z is going to be the mean of X plus the mean of Y. Or another way to view it is if I wanted to take, let's say I have some other random variable.