While using the Ajaxtoolkit and if you have different version of Framework you might receive this message

Error Message :

Error 2 Assembly 'AjaxControlToolkit, Version=3.0.30512.17815, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' uses 'System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' which has a higher version than referenced assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'

Solution :

in the web config file inside the configuration section add this tag