Physics. I have absolutely no clue how to do this. please help me.?

One gallon of gas contains about 1 x 108 joules of energy. A 1000-kg car traveling at 20 m/s brakes and comes to a stop. Estimate how much gasoline it will take to bring the car back to the original speed. Factor in that only about 20% of the energy extracted from gasoline actually propels the car. The rest gets exhausted as heat and unburnt fuel. Explain how you came up with your answer.