[myhanh]

a[i].tong:=0;
i:=1;
dem:=0;
Write('Nhap vao a[',i,']:');readln(a[i].hoten);
Write('Toan:');Readln(a[i].Toan);
Write('Li  :');Readln(a[i].Li);
Write('Hoa :');Readln(a[i].Hoa);
a[i].tong:=a[i].tong+a[i].toan+a[i].li+a[i].hoa;
While a[i].hoten<>'' do begin
 dem:=dem+1;
 i:=i+1;
  Write('Nhap vao a[',i,']:'); readln(a[i].hoten);
  Write('Toan:');Readln(a[i].Toan);
  Write('Li  :');Readln(a[i].Li);
  Write('Hoa :');Readln(a[i].Hoa);
 a[i].tong:=a[i].tong+a[i].toan+a[i].li+a[i].hoa;
   end;

Em chỉ cần kiểm tra một tí:

a[i].tong:=0;
 i:=1;
 dem:=0;
 Write('Nhap vao a[',i,']:');readln(a[i].hoten);
 Write('Toan:');Readln(a[i].Toan);
 Write('Li  :');Readln(a[i].Li);
 Write('Hoa :');Readln(a[i].Hoa);
 a[i].tong:=a[i].tong+a[i].toan+a[i].li+a[i].hoa;
 While a[i].hoten<>'' do begin
  dem:=dem+1;
  i:=i+1;
   Write('Nhap vao a[',i,']:'); readln(a[i].hoten);
if a[i].hoten<>'' then
begin
   Write('Toan:');Readln(a[i].Toan);
   Write('Li  :');Readln(a[i].Li);
   Write('Hoa :');Readln(a[i].Hoa);
  a[i].tong:=a[i].tong+a[i].toan+a[i].li+a[i].hoa
end
    end;

Chúc em thành công nha!