Interview question [entry-level,java,finance]: solve and comment on what's wrong with this sample

TaxMan.java

// Calculate the tax from the amount as you would do in a real world scenario.
class TaxMan{

  //Return the tax as percentage of the amount plus penalty
  private static double calcTaxes(long amount, int perc, double penalty){
    //fill here
  }


  public static void main(String args...){
    System.out.println(calcTaxes(1,10,0.1) == 0.2);
    System.out.println(calcTaxes(1,20,0.02) == 0.22);
    System.out.println(calcTaxes(1,5,0) == 0.05);
    System.out.println(calcTaxes(1,20,0.01) == 0.21); //If it breaks here, explain why.
    System.out.println(calcTaxes(1,10,0) == 0.1);
  }

}