Câu 2 :
uses crt;
var f1, f2 : text; M, N, S, i : integer;
begin
clrscr;
assign(f1, 'DULIEU.TXT'); reset(f1);
assign(f2, 'KETQUA.TXT'); rewrite(f2);
read(f1, M, N); S := 0;
for i := M to N do
if i mod 3 = 0 then S := S + i;
write(f2, S);
close(f1); close(f2);
readln
end.