A good answer might be:

The class is completed below.

Adding the Remaining Class

The constant taxRate is used in the calculateTax() method just as if it had been defined in the Toy class. Also, it can be used in methods of the class other than those listed in the interface. 

class Toy extends Goods implements Taxable
{
  int minimumAge;

  Toy( String des, double pr, int min)
  {
    super( des, pr );
    minimumAge  = min ;
  }

  void display()
  {
    super.display() ;
    System.out.println( "minimum age: " + minimumAge );
  }

  public double calculateTax()
  {
    return price * taxRate ;
  }
}

The CalculateTax() method must be made public.

QUESTION 10:

Can another class implement the Taxable interface?

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