Car.java

public class Car {
    public double efficiency;

    public double carGas;

    public double numberOfDistancePerLitter;

    Car(double efficiency){
        this.efficiency = efficiency;
    }
    public void addGas(double gas){
        this.carGas = gas;
        this.numberOfDistancePerLitter = this.carGas * this.efficiency;
    }

    public double getGasInTank(){
        double carGas = this.numberOfDistancePerLitter / this.efficiency;
        return carGas;
    }

    public void drive(double distance){
        this.numberOfDistancePerLitter -= distance;
    }
}

CarTester.java

public class CarTester {
    public static void main(String[] args){
        Car myCar = new Car(20);	// 20 kilometers per litter
        myCar.addGas(40);		// Add gasoline 40 litters
        myCar.drive(100);		// Drive 100 kilometers
        double gasLeft = myCar.getGasInTank();
        System.out.println(gasLeft);
    }
}