4) One gallon of petrol contains the same amount of energy as a kg of hydrogen. However, fuel cell electric vehicles have a better energy efficiency and can travel 96 km per kilogram of hydrogen. With this information, determine how many billion kg of hydrogen would be needed if all the vehicles in USA are replaced by the fuel cell electric vehicles. Assumptions: There are 200 million vehicles in USA. Average annual travel mileage of a vehicle in USA is 12,000 miles/car/year. Average gas mileage of vehicles in USA is 20 miles/gal (US gallon).The density of petrol is 0.737 g/cm³.​