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. And only because they are independent can we add the variances. So what you can say is that the variance of the combined trip is equal to the variance of going to work plus the variance of going home. So what's the variance of going to work?