A gallon of gasoline contains about 1.3x10^8 joules of energy. A 2000-kg car traveling 20 m/s skids to a stop. Estimate how much gasoline it will take to bring the car back to the original speed. To complicate matters further, consider the fact that only about 15% of the energy extracted from gasoline actually propels the car. The rest gets exhausted as heat and unburnt fuel. Explain your answer in depth using words and equations.