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>OK, I have put this in the head section, but it is telling me it cannot find jquery.js, even though this file is in the same folder as the master file. Where should it go?

That path is relative to the ASPX page you are viewing. If your master page is in the "root" of the web app, change the script path to "/" + script name. If your master file is in a subdirectory you will need to provide a relative path to that directory, ex. /MasterPages/jquery.js

Those are two different Javascript files. The core jQuery file (jquery.js) and the plug-in for input masks.