In what situation you would use multiple version of jQuery and how would you include them?

Answer

Well, it is quite possible that the jQuery plugins which are used are dependent on older version but for your own jQuery code, you would like to use newer version. So because of this dependency, multiple version of jQuery may require sometimes on single page.  
Below code shows how to include multiple version of jQuery.

<script type='text/javascript' src='js/jquery_1.9.1.min.js'></script>  
<script type='text/javascript'>  var $jq = jQuery.noConflict(); </script>  
<script type='text/javascript' src='js/jquery_1.7.2.min.js'></script>  
By this way, for your own jQuery code use "$jq", instead of "$" as "$jq" refers to jQuery 1.9.1, where "$" refers to 1.7.2.