type TR=Record
        Name:string[100]; 
        Age:Byte; 
        Income:Real; 
       end; 
var f:file of TR; 
    r:TR; 

begin 
//assign file 
  assignFile(f, 'MyFileName'); 
//open file 
  if FileExists('MyFileName') then 
    reset(f) 
  else 
    rewrite(f); 
//чтение 10й записи 
  seek(f,10); 
  read(f,r); 
//запись 20й записи 
 seek(f, 20); 
 write(f,r); 
 closefile(f); 
end;