Suppose a car travels 108 km at a speed of 40.0 m/s, and uses 2.00 gallons of gasoline. Only 30% of the gasoline goes into useful work by the force that keeps the car moving at constant speed despite friction. (The energy content of gasoline is 1.30 x 10 ] per gallon.) (a) What is the force (in N) exerted to keep the car moving at constant speed? (Enter a number.) N (b) If the required force is directly proportional to speed, how many gallons will be used to drive 108 km at a speed of 28.0 m/s? (Enter a number.) gallons