[Paulao]

Ahora hizo mi calculadora y quedo bueno. Abajo mis codigos: Esta es la Unit donde yo implente mis classes.

Código Delphi [-]
unit Unit2;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TCalculadora = class
  private
    FOp1, FOp2: integer;
    procedure SetOp1(Value: integer);
    procedure SetOp2(Value: integer);
    function GetOp1: integer;
    function GetOp2: integer;
  public
    property Op1: integer read GetOp1 write SetOp1;
    property Op2: integer read GetOp2 write SetOp2;
    function Soma: integer;
    function Subtrai: integer;
    function Multiplica: integer;
    function Divide: double;
    function Fatora: integer;
    function Resta: integer;

end;

implementation

{ TCalculadora }

function TCalculadora.Divide: double;
begin
  result := FOp1 / FOp2;
end;

function TCalculadora.Fatora: integer;
var
  N,i:Integer;
begin

  N := FOp2;
  for i := 1 to N - 1 do
    N := N * i;
  result := N;
end;

function TCalculadora.GetOp1: integer;
begin
  Result := FOp1;
end;

function TCalculadora.GetOp2: integer;
begin
  Result := FOp2;
end;

function TCalculadora.Multiplica: integer;
var
  i,j:Integer;
begin
  i := 1;
  j := FOp1;
  while i < FOp2 do
  begin
    FOp1 := Fop1 + j;
    inc(i);
  end;
  Result := FOp1;
end;

function TCalculadora.Resta: integer;
begin

end;

procedure TCalculadora.SetOp1(Value: integer);
begin
  FOp1 := Value;
end;

procedure TCalculadora.SetOp2(Value: integer);
begin
  FOp2 := Value;
end;

function TCalculadora.Soma: integer;
begin
  result := FOp1 + FOp2;
end;

function TCalculadora.Subtrai: integer;
begin
  result := Fop1 - Fop2;
end;

end.

Ahora otra Unit donde este mi llamada a la classe

Código Delphi [-]
unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, Unit2, StdCtrls, Buttons;

type
  TForm1 = class(TForm)
    Edit1: TEdit;
    Edit2: TEdit;
    Edit3: TEdit;
    BitBtn1: TBitBtn;
    BitBtn2: TBitBtn;
    BitBtn3: TBitBtn;
    BitBtn4: TBitBtn;
    BitBtn5: TBitBtn;
    procedure BitBtn1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure BitBtn2Click(Sender: TObject);
    procedure BitBtn3Click(Sender: TObject);
    procedure BitBtn4Click(Sender: TObject);
    procedure BitBtn5Click(Sender: TObject);
  private
    { Private declarations }
    FCalc: TCalculadora;
  public
    { Public declarations }
    property Calc: TCalculadora read FCalc write FCalc;
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.BitBtn1Click(Sender: TObject);
begin
  FCalc.Op1 := StrToInt(Edit1.Text);
  FCalc.Op2 := StrToInt(Edit2.Text);
  Edit3.Text := IntToStr(Calc.Soma);
end;

procedure TForm1.BitBtn2Click(Sender: TObject);
begin
  FCalc.Op1 := StrToInt(Edit1.Text);
  FCalc.Op2 := StrToInt(Edit2.Text);
  Edit3.Text := IntToStr(Calc.Subtrai);
end;

procedure TForm1.BitBtn3Click(Sender: TObject);
begin
  FCalc.Op1 := StrToInt(Edit1.Text);
  FCalc.Op2 := StrToInt(Edit2.Text);
  Edit3.Text := IntToStr(Calc.Multiplica);
end;

procedure TForm1.BitBtn4Click(Sender: TObject);
begin
  FCalc.Op1 := StrToInt(Edit1.Text);
  FCalc.Op2 := StrToInt(Edit2.Text);
  Edit3.Text := FloatToStr(Calc.Divide);
end;

procedure TForm1.BitBtn5Click(Sender: TObject);
begin
  FCalc.Op1 := StrToInt(Edit1.Text);
  FCalc.Op2 := StrToInt(Edit2.Text);
  Edit3.Text := IntToStr(Calc.Fatora);
end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  FreeAndNil(FCalc);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  FCalc := TCalculadora.Create;
end;

end.

Esta funciono con poco codigo, que es mi abjetivo.