The type or namespace name Linq does not exist in the namespace

Error 1 The type or namespace name 'Linq' does not exist in the namespace 'System' (are you missing an assembly reference?)

Solution:
Make sure to include the reference to system.core dll ( version 3.5.0.0)

 <system.web>
    <compilation debug="false">
      <assemblies>       
 <add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
 </assemblies>
    </compilation>
 </system.web>