This is a problem in basic algebra. You are given (you ask the user for)

• The number of gallons of gas in the tank
• The fuel efficiency in miles per gallon
• The price of gas per gallon

and asked to find two things: 

• cost per 100 miles and
• how far the car can go with the gas in the tank.

Compute each one separately and print the result. 

Here is an outline of a class you might use.

/**
 * A Car class for exercise E 4.10
 */
public class Car4_10
{
    // instance variables
    private double gallonsInTank;
    private double fuelEfficiency;
    private double pricePerGallon;

    /**
     * Constructor for objects of class Car4_10
     */
    public Car4_10(double gallonsInTank, double fuelEfficiency, double pricePerGallon)
    {
        // initialize instance variables
  
    }

    public double getCostPerMile()
    {
        return /* compute cost per mile */
    }

    public double getDistanceTilEmpty()
    {
        return /* compute distance until empty */
    }

}

Here is a sketch of a JUnit test method you might use.

    @Test
    public void testE4_10() {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Gallons in tank: ");
        double gallonsInTank = scanner.nextDouble();
        // similarly for fuelEfficiency and pricePerGallon

        Car4_10 car = new Car4_10(gallonsInTank, fuelEfficiency, pricePerGallon);
        System.out.print("Expected cost for 100 miles: ");
        double expectedCostFor100Miles = scanner.nextDouble();
        assertEquals(expectedCostFor100Miles, 100 * car.getCostPerMile(), 0.01);
        // similarly for distance 'til empty 
    }