Last week Fred had 12 dollars and Jason had 24 dollars. Fred washed cars over the weekend and now has 25 dollars. Jason delivered newspapers and now has 7 dollars. How much money did they earn by washing cars?
Fred had 12 dollars and Jason had 24 dollars. 

Fred washed cars over the weekend and now has 25 dollars. 

Jason delivered newspapers and now has 7 dollars. 

We want to know how much money Fred earned by washing cars. 

We can figure this out by looking at the difference in Fred's money before and after he washed cars. 

Before he washed cars, he had 12 dollars. 

After he washed cars, he had 25 dollars. 

This means that he earned 13 dollars by washing cars.