How to resolve error Assembly 'AjaxControlToolkit has a higher version than System.Web.Extensions, Version=1.0.61025.0 referenced assembly

Error 1 Assembly 'AjaxControlToolkit, Version=3.5.40412.0, 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'


This error occurs when there is mismatch in AJAXControlToolkit version and System.Web.extensions version.
You can resolve this error without upgrading your System.web.extensions version bu following simple steps.

1. In Solution Explorer Click and Open Web.config file
2. Replace this Web.config file with this new Web.config  file .
3. <?xml version="1.0"?>
4. <configuration>
5.   <configSections>
6.     <sectionGroup name="system.web.extensions" type="System.Web.Configuration.SystemWebExtensionsSectionGroup, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
7.       <sectionGroup name="scripting" type="System.Web.Configuration.ScriptingSectionGroup, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
8.         <section name="scriptResourceHandler" type="System.Web.Configuration.ScriptingScriptResourceHandlerSection, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" allowDefinition="MachineToApplication"/>
9.         <sectionGroup name="webServices" type="System.Web.Configuration.ScriptingWebServicesSectionGroup, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
10.           <section name="jsonSerialization" type="System.Web.Configuration.ScriptingJsonSerializationSection, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" allowDefinition="Everywhere"/>
11.           <section name="profileService" type="System.Web.Configuration.ScriptingProfileServiceSection, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" allowDefinition="MachineToApplication"/>
12.           <section name="authenticationService" type="System.Web.Configuration.ScriptingAuthenticationServiceSection, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" allowDefinition="MachineToApplication"/>
13.         </sectionGroup>
14.       </sectionGroup>
15.     </sectionGroup>
16.   </configSections>
17.   <system.web>
18.     <pages>
19.       <controls>
20.         <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
21.       </controls>
22.     </pages>
23.     <!--
24.           Set compilation debug="true" to insert debugging
25.           symbols into the compiled page. Because this
26.           affects performance, set this value to true only
27.           during development.
28.     -->
29.     <compilation debug="true">
30.       <assemblies>
31.         <add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
32.         <add assembly="System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
33.         <add assembly="System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
34.         <add assembly="System.Web.Extensions.Design, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
35.       </assemblies>
36.     </compilation>
37.     <httpHandlers>
38.       <remove verb="*" path="*.asmx"/>
39.       <add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
40.       <add verb="*" path="*_AppService.axd" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
41.       <add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false"/>
42.     </httpHandlers>
43.     <httpModules>
44.       <add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
45.     </httpModules>
46.   </system.web>
47.   <runtime>
48.     <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
49.       <dependentAssembly>
50.         <assemblyIdentity name="System.Web.Extensions" publicKeyToken="31BF3856AD364E35" culture="neutral"/>
51.         <bindingRedirect oldVersion="1.0.61025.0" newVersion="3.5.0.0"/>
52.       </dependentAssembly>
53.       <dependentAssembly>
54.         <assemblyIdentity name="System.Web.Extensions" publicKeyToken="31BF3856AD364E35" culture="neutral"/>
55.         <bindingRedirect oldVersion="2.0.0.0" newVersion="3.5.0.0"/>
56.       </dependentAssembly>
57.     </assemblyBinding>
58.   </runtime>
59.   <system.web.extensions>
60.     <scripting>
61.       <webServices>
62.         <!-- Uncomment this line to customize maxJsonLength and add a custom converter -->
63.         <!--
64.       <jsonSerialization maxJsonLength="500">
65.         <converters>
66.           <add name="ConvertMe" type="Acme.SubAcme.ConvertMeTypeConverter"/>
67.         </converters>
68.       </jsonSerialization>
69.       -->
70.         <!-- Uncomment this line to enable the authentication service. Include requireSSL="true" if appropriate. -->
71.         <!--
72.         <authenticationService enabled="true" requireSSL = "true|false"/>
73.       -->
74.         <!-- Uncomment these lines to enable the profile service. To allow profile properties to be retrieved
75.            and modified in ASP.NET AJAX applications, you need to add each property name to the readAccessProperties and
76.            writeAccessProperties attributes. -->
77.         <!--
78.       <profileService enabled="true"
79.                       readAccessProperties="propertyname1,propertyname2"
80.                       writeAccessProperties="propertyname1,propertyname2" />
81.       -->
82.       </webServices>
83.       <!--
84.       <scriptResourceHandler enableCompression="true" enableCaching="true" />
85.       -->
86.     </scripting>
87.   </system.web.extensions>
88.   <system.webServer>
89.     <validation validateIntegratedModeConfiguration="false"/>
90.     <modules>
91.       <add name="ScriptModule" preCondition="integratedMode" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
92.     </modules>
93.     <handlers>
94.       <remove name="WebServiceHandlerFactory-Integrated"/>
95.       <add name="ScriptHandlerFactory" verb="*" path="*.asmx" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
96.       <add name="ScriptHandlerFactoryAppServices" verb="*" path="*_AppService.axd" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
97.       <add name="ScriptResource" preCondition="integratedMode" verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
98.     </handlers>
99.   </system.webServer>
100. </configuration>
101. Copy the above code in Web.config file save it debug and run .