The MFT of C drive may have problem. This is why that shows 48GB instead 298GB.
Use this command to fix that problem:(on cmd)
chkdsk C: /f/r/x
and press
'Y'
when it ask.
It may resolve MFT problem.
320-298 = 22GB was used for Partition and invisible to user.
What is MFT?
MFT is Master File Table. What happen when you create an empty folder (named 'New Folder' here is 10 character=10 byte) that information is stored in MFT. This is why MFT takes some space from HDD even there is not a single file/folder.