Exercise 1: add back to top only after 4th paragraph.

Chapter 5/05.js

@@ -13,7 +13,7 @@ $(document).ready(function () {
   });
 
   // Add "back to top" links.
-  $('<a href="#top">back to top</a>').insertAfter('div.chapter p');
+  $('div.chapter p').slice(4).after($('<a href="#top">back to top</a>'));
   $('<a id="top"></a>').prependTo('body');
   // insertBefore <elem> prependTo <children /> appendTo </elem> insertAfter