Lời giải :
program hotrotinhoc ;
var a : array[1..100] of integer ;
i,n,dem,j : integer ;
begin
write('Nhap n='); readln(n);
writeln('Nhap gia tri cua cac phan tu');
for i:= 1 to n do
begin
write('a[',i,]='); readln(a[i]);
end;
dem:=0;
for i:= 1 to n do
for j:= 30 to 39 do
if a[i] = j then dem:=dem+1;
write('Co tat ca ',dem,' so co chu so hang chuc la 3');
readln
end.