You may like to have a look at the Excel interactivity based on
  this problem <a href=
  "http://nrich.maths.org/public/viewer.php?obj_id=5405&amp;part=index">
  here</a> .<a href=
  "http://nrich.maths.org/public/viewer.php?obj_id=5403&amp;part=null">
  </a> <br />