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stanley drove his car on a business trip when he left, the mileage was 840 miles, and when he returned, the mileage was 1,200 miles. the car used 12 gallons of gasoline for this trip. carla made the same trip as stanley, but her car used only 10 gallons of gasoline. how do the slopes for stanley's and carla's cars compare?

The slopes of the cars are calculated by dividing the number of miles driven by the amount of gasoline consumed for the trip. 
(Carla) 
                       s = (1200 miles - 840) / 12 gal = 30 miles/gal
(Stanley)
                       s = (1200 miles - 840) / 10 gal = 36 miles/gal
Therefore, Stanley's car has a greater slope which means that for the same amount of gasoline, Stanley's car can travel farther.