- Dùng lệnh For .. do :

uses crt;

var i, N, S : integer;

begin

clrscr;

write('Nhap so N = '); readln(N);

S := 0; for i := 1 to N do

if i mod 3 = 0 then S := S + i;

write('Tong cac so chia het cho 3 = ', S);

readln

end.

- Dùng lệnh While .. do :

uses crt;

var i, N, S : integer;

begin

clrscr;

write('Nhap so N = '); readln(N);

S := 0; i := 1;

while i <= N do begin

if i mod 3 = 0 then S := S + i;

i := i + 1; end;

write('Tong cac so chia het cho 3 = ', S);

readln

end.

FOR...DO:

uses crt;

var i,s,n: integer;

begin

clrscr;

write('Nhap n: ');

readln(n);

for i:=1 to n do if i mod 3 = 0 then s:=s+i;

write('Tong cac so chia het cho 3 la: ',s);

readln

end.

WHILE...DO:

uses crt;

var i,s,n: integer;

begin

clrscr;

write('Nhap n: ');

readln(n);

while i<=n do

begin

if i mod 3 = 0 then s:=s+i;

inc(i);

end;

write('Tong cac so chia het cho 3 la: ',s);

readln

end.