public double cityRent(Bicycle[] list) {
    double result = 0.0;
    for (int i = 0; i < list.length; i++) {
      if (list[i] instanceof CityBike) 
        result += list[i].getAmount(4);
    }//for
    return result;
  }
  
  public double sportsRent(Bicycle[] list) {
    double result = 0.0;
    for (int i = 0; i < list.length; i++) {
      if (list[i] instanceof SportsBike) 
        result += list[i].getAmount(4);
    }//for
    return result;
  }
  
  public double totalRent(Bicycle[] list) {
    double result = 0.0;
    for (int i = 0; i < list.length; i++) {
      result += list[i].getAmount(4);
    }//for
    return result;
  }
  
  public int noWithSeat(Bicycle[] list) {
    int result = 0;
    for (int i = 0; i < list.length; i++) {
      if (list[i] instanceof CityBike) {
        if ( ((CityBike) list[i] ).getHasBabySeat() ) 
          result++;
      }//if CityBike
    }//for
    return result;
  }
  
  public double averageGears(Bicycle[] list) {
    int cycles = 0;
    double gears = 0.0;
    for (int i = 0; i < list.length; i++) {
      if (list[i] instanceof SportsBike) {
        cycles++;
        gears += ( (SportsBike) list[i] ).getGears();
      }//if SportsBike
    }//for
    return gears / cycles;
  }