Here's an example question:
When I drive at an average speed of 45 mph I can get 6 miles more to the gallon than I can when I drive at an average speed of 60 mph. The other day I drove to a destination at 45 mph average and the day after I drove back at 60 mph average, and my total travelling time was 21 hours. How far away was my destination, and how much fuel did I use over the whole journey to and from the destination if my fuel consumption at 60 mph is 30 miles per gallon?