Dùng repeat until
program nhapso;
uses crt;
var x:integer;
begin
clrscr;
repeat
write('Nhap x: '); readln(x);
until (x>10) and (x<20);
readln;
end.
Dùng while...do
program nhapso;
uses crt;
var x:integer;
begin
clrscr;
x:=0;
while (x<=10) or (x>=20) do
begin
write('Nhap so: '); readln(x);
end;
readln;
end.