Last week Fred had w dollars and Jason had x dollars. Over the weekend Fred delivered newspapers earning y dollars and washed cars earning z dollars. How much money did Fred earn over the weekend?
First, let's think about what we know. We know that Fred had w dollars last week, and we know that Jason had x dollars last week. So we can write: 

Fred: w 
Jason: x 

Now we need to think about what happened over the weekend. We know that Fred delivered newspapers and washed cars. So we can write: 

Fred: w + y + z 
Jason: x

We can see that Fred earned y + z dollars over the weekend.