Easy modification to jQuery code to make use of more than one textbox
<p>How to change the following code to make use of all my 4 textboxes with id extra1, extra2,persons and tables?</p>

<pre><code>$(document).ready(function () {
  $('#extra1').blur(function () {
        if ($.trim(this.value) == "") {
          $('#btnUpdate').attr("disabled", true);
        }
      else {
            $('#btnUpdate').removeAttr("disabled");
      }
    });
});
</code></pre>
