So what we wanna do is come up with a total distribution, home and back, I guess you could say. We could call this work plus home, home and back. If you have two random variables that can be described by normal distributions, and you were to define a new random variable as their sum, the distribution of that new random variable will still be a normal distribution, and its mean will be the sum of the means of those other random variables. So the mean here, I'll say the mean of work plus home, is going to be equal to 20 liters. He will use a mean of 20 liters in the round trip. Now for the standard deviation from home plus work, you can't just add the standard deviations, going and coming back. But because the amount of fuel going to work and the amount of fuel coming home are independent random variables, because they are independent of each other, we can add the variances.