Computers in some vehicles calculate various quantities related to performance . One of these is the efficiency , or gas mileage , usually expressed as miles per gallon ( may ) . For one vehicle equipped in way the miles per gallon were recorded each time the gas tank was filled , and the computer was it reset In addition to the computer's calculations of miles per gallon , the driver also recorded the m per gallon by dividing the miles driven by the number of gallons at each fill-up . The following data the differences between the computer's and the driver's calculations for that random sample of 20 records . The driver wants to determine if these calculations are different Assume that the standard deviation of a difference is J = 30