Stanley drove his car on a business trip when he left the mleage was 840miles. and. when he returned the mileage was 1,200miles. the car used 12 gallons of gasoline for this trip

I think for this item, the rate at which the gasoline was consumed is to be calculated. To determine the distance that Stanley's car had traveled, we subtract 840 miles from 1,200 miles. That will give us an answer of 360 miles. Dividing this by 12 gallons will give us an answer of 30 miles/gal. 

Using the rise/run rule. you would get 2/6.simp. 1/3