[The Shadow]

Вот рабочий пример со StringGrid'ом:

Код:

unit Unit1;
interface
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, Grids;
type
  TForm1 = class(TForm)
    StringGrid1: TStringGrid;
    procedure FormCreate(Sender: TObject);
    procedure StringGrid1SetEditText(Sender: TObject; ACol, ARow: Integer;
      const Value: String);
    procedure StringGrid1GetEditText(Sender: TObject; ACol, ARow: Integer;
      var Value: String);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
var
  Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
begin
with StringGrid1 do
begin
Cells[0, 0]:='0';
Cells[1, 0]:='0';
Cells[2, 0]:='0';
end;
end;
procedure TForm1.StringGrid1SetEditText(Sender: TObject; ACol,
  ARow: Integer; const Value: String);
begin
try
StringGrid1.Cells[2, 0]:=IntToStr(StrToInt(StringGrid1.Cells[0, 0]) + StrToInt(StringGrid1.Cells[1, 0]));
except
end;
end;
procedure TForm1.StringGrid1GetEditText(Sender: TObject; ACol,
  ARow: Integer; var Value: String);
begin
if ACol = 2 then
Exit;
try
StrToInt(Value);
except
Value:='0';
end;
StringGrid1.Cells[2, 0]:=IntToStr(StrToInt(StringGrid1.Cells[0, 0]) + StrToInt(StringGrid1.Cells[1, 0]));
end;
end.

В StringGrid'e должно быть три столбца, и одна строка, в Option - goEditing поставь True, и не забудь указать события OnCrete у формы, и события OnGetEditText и OnSetEditText у StringGrid'a