Error: unresolved dependencies

Problem
Sometimes installing new packages or upgrading existing packages using Synaptic Package Manager results in an error, something like:

Could not mark all packages for installation or update: the following packages have unresolved dependencies. Make sure that all required repositories are added and enabled in the preferences

skype: Depends: libqtcore4 but it is not installable Depends: libqtdbus4 but it is not installable Depends: libqtgui4 but it is not installable Depends: libqtnetwork4 but it is not installable Depends: libQtDBus.so.4 but it is not installable

Solution
In most probability, you just need to open the testing section of either Granular or PCLinuxOS repository.