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I installed Kubuntu 12.04 on this desktop last September and am the main user of the machine. I can perform most admin tasks such as installing new software with my password, which is also accepted using
sudo in a terminal. Some tasks however ask for a root password - the most recent example being when I tried to re-install my HP printer. and my usual password does not work. I have looked at this answer, but it doesn't seem to help as I know my own password. I need to know root's password!