For A, divide total miles by the miles per gallon for each tire pressure to find the number of gallons of gas used:
At 30 psi: 23000 / 20.5 = 1121.95 gallons , round to 1121 gallons.
At 35 psi: 23000 / 21.1 = 1090 gallons.
Number of gallons saved: 1121 - 1090 = 31 gallons.
B) multiply the cost per gallon by the number of gallons saved:
31 gallons x $2.00 = $62.00 saved.
C) Multiply the gallons saved by 1 car by total number of cars:
31 gallons x 140 million cars = 4,340 million gallons ( 4 billion 340 million gallons)