how to select files which is not accessed before 20days in linq

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LINQ

hi,
var Files = Directory.GetFiles(SourcePath).Select(x => new FileInfo(x)).OrderByDescending(x => x.LastAccessTime).Take(20).ToArray();


here i am getting last accessed 20 files,


but i wanted file before 20 days, there might be 100 files before 20 days i should get tht 100 files , pls help me with this using linq