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

There are few things have to notice for above error, and do and check one by one.

1. Check that you are using correct version of ASP.Net in IIS( Check applicationpool ),if not select exact one. 
2. If your ASP.NET  Version is 3.5, Ajax dll inbuilted in Framework 3.5. So change your extension version to 3.5 in web confiq.
3. If your asp.net version is 2.0 you have to install ASPNET.2.0.AJAX.Extensions on your server.

Here i have given the link to download ASPNET.2.0.AJAX.Extensions.

4. Redirect old version to new version like below.
 

<runtime>
 <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
  <dependentAssembly>
   <assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35" />
   <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
  </dependentAssembly>
  <dependentAssembly>
   <assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35" />
   <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
  </dependentAssembly>
 </assemblyBinding>
</runtime>

5. Change public key version to 31BF3856AD364E35(its work, how ? me too)

I hope this will help you.