Follwing the steps below to find out the version of .NET Framework installed in your Windows system:
Using Windows Explorer
All Microsoft .NET Framework versions are installed in following directory:
%windir%\Microsoft.NET\Framework\
Where "%windir%" represents "Windows" directory present in the system drive where Windows is installed in your system e.g. C:\Windows\.
So just type the above mentioned path in RUN dialog box or in Windows Explorer addressbar and press Enter. It'll open the directory as shown in following screenshot:
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Here in this folder, you can find out which versions are installed.