When a​ car's tire pressure is 30 pounds per square inch​ (psi), it averages 20.7 mpg of gasoline. If the tire pressure is increased to 35​ psi, the car averages 21.1 mpg of gasoline. ​a) If someone drives an average of 17,000 mi per​ year, how many gallons of gasoline will he save in a year by increasing his tire pressure from 30 to 35​ psi? b) If gasoline costs ​$3.00 per​ gallon, how much will he save in a​ year? c) Assume that there are about 100 million cars in a country and that these changes are typical of each​ car, how many gallons of gasoline would be saved if all drivers increased their​ cars' tire​ pressure?