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qw@ubuntu:~ $ sudo apt-get install virtualbox

password for qw [sudo]:

Reading lists of packages... It is ready

Creation of a tree of dependences      

Reading information on a condition... It is ready

The virtualbox package is inaccessible, but will mention in the list of dependences of other package.

It can mean that the package is absent, obsolete, or is available from the sources which have not been mentioned in sources.list

E: For a virtualbox package candidates for installation aren't found

qw@ubuntu:~ $