Var n:int64;

Function T(n:int64):lint64;

  Begin

   if n<10 then t:=n else T:=T(n div 10)+T(n mod 10);

End;

Begin

 write('nhap n='); read(n);

 write('tong ',t(n)); readln end.

Dùng function:

uses crt;
var n:longint;
function tong(a:longint):longint;
begin
   tong:=0;
   repeat
      inc(tong,a mod 10);
      a:=a div 10;
   until a=0;
end;
begin
clrscr;
   write('Nhap mot so: ');readln(n); 
   writeln('Tong cac chu so cua so do: ',tong(n));
readln
end.

Không dùng function:

uses crt;
var n,tong:longint;
begin
clrscr;
   write('Nhap mot so: ');readln(n); 
   tong:=0;
   repeat
      inc(tong,n mod 10);
      n:=n div 10;
   until n=0;
   writeln('Tong cac chu so cua so do: ',tong);
readln
end.