Could not load file or assembly 'System.Web.Helpers' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference.

Could not load file orassembly 'System.Web.Helpers' 
or one of its dependencies. The locatedassembly's 
manifest definition does not match the assembly reference.
(Exception from HRESULT: 0x80131040)
Description: Anunhandled exception occurred during the execution of the current web request.Please review t
he stack trace for more information about the error and where itoriginated in the code. 
Exception details:System.IO.FileLoadException:Could not load file or assembly 'System.Web.Helpers' 
or one of itsdependencies. The located assembly's manifest definition does not match the 
assembly reference. (Exception from HRESULT: 0x80131040)

You need to Uninstall Microsoft Aspnet mvc from the project by using Package manager console and reinstall it.

Uninstall-Package Mictrosoft.AspNet.Mvc

You can also remove the installed package by deleting the following command from the package.config.

<package id="Microsoft.AspNet.Razor" version="3.2.3" targetFramework="net45" />

Reinstall package by using the command, 
 

PM> Install-Package Mictrosoft.AspNet.Mvc