Question:

Could not load file or assembly 'System.Web.Extensions,
Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' 
or one of its dependencies. The system cannot find the file specified. 

solution:

1.Go to Add reference, Select System.Web.Extensions and install this to WebConfig.
and replace existing one in web config.

<compilation debug="true">
<assemblies>
<--replace instead of this -->
<add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>


<--with this one-->


<add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>


</assemblies>
</compilation>