#100% đúng.

uses crt;
var s,h:string; i,d,t,kt,j:longint;
begin
clrscr;
   write('Nhap 1 xau: ');readln(s);
   write('Xau theo chieu nguoc lai: ');
   for i:=length(s) downto 1 do write(s[i]);
   writeln;
   write('Chu cai cua xau: ');
   for i:=1 to length(s) do if (s[i] in ['a'..'z'])or(s[i] in ['A'..'Z']) then
   write(s[i]); writeln;
   h:='abc';
   for i:=1 to length(s) do
      begin
         kt:=0;
         for j:=i to i+2 do if s[j]<>h[j-i+1] then kt:=1;
         if kt=0 then inc(d);
      end;
   write('Xau "abc" xuat hien ',d,' lan trong xau vua nhap');
readln
end.

Xin hay nhất

program xaunha;

uses crt;

var st:string;

      i,d:byte;

begin

clrscr;

write('Nhap xau: ');

readln(st);

write('Xau dao nguoc la: ');

for i:=length(st) downto 1 do

  write(st[i]);

writeln;

write('Cac ki tu la chu cai la: ');

for i:=1 to length(st) do

  if (st[i] in ['a'..'z']) or (st[i] in ['A'..'Z']) then write(st[i]);

writeln;

d:=0;

while pos('abc',st)<>0 do

  begin

   inc(d);

   delete(st,pos('abc',st),3);

  end;

write('So lan xuat hien cua abc la: ',d);

readln

end.