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I am unable to install Virtualbox5.1 from oracle ppa; with the following error

The following packages have unmet dependencies:

 virtualbox-5.1 : Depends: libqt5core5a (>= 5.6.0~beta) but 5.5.1+dfsg-16ubuntu7.2 is to be installed

                  Depends: libqt5widgets5 (>= 5.6.0~beta) but 5.5.1+dfsg-16ubuntu7.2 is to be installed

                  Depends: libqt5x11extras5 (>= 5.6.0) but 5.5.1-3build1 is to be installed

                  Recommends: libsdl-ttf2.0-0 but it is not going to be installed

E: Unable to correct problems, you have held broken packages.

These are all dependencies for most recent VirtualBox 5.1.18 installed from the Oracle repository for Ubuntu 16.04.2 LTS:

Dependencies:

5.1.18-114002~Ubuntu~xenial - debconf (18 1.1) debconf-2.0 (0 (null)) libc6 (2 2.15) libcurl3 (2 7.16.2) libdevmapper1.02.1 (2 2:1.02.97) libgcc1 (2 1:3.0) libgl1-mesa-glx (16 (null)) libgl1 (0 (null)) libpng12-0 (2 1.2.13-4) libpython2.7 (2 2.7) libqt5core5a (2 5.5.0) libqt5gui5 (18 5.3.0) libqt5gui5-gles (2 5.3.0) libqt5opengl5 (18 5.0.2) libqt5opengl5-gles (2 5.0.2) libqt5printsupport5 (2 5.0.2) libqt5widgets5 (2 5.2.0) libqt5x11extras5 (2 5.1.0) libsdl1.2debian (2 1.2.11) libssl1.0.0 (2 1.0.0) libstdc++6 (2 5.2) libvpx3 (2 1.5.0) libx11-6 (0 (null)) libxcb1 (0 (null)) libxcursor1 (4 1.1.2) libxext6 (0 (null)) libxinerama1 (0 (null)) libxml2 (2 2.7.4) libxmu6 (0 (null)) libxt6 (0 (null)) zlib1g (2 1:1.1.4) psmisc (0 (null)) adduser (0 (null)) virtualbox (0 (null)) virtualbox:i386 (0 (null)) virtualbox-guest-additions-iso (0 (null)) virtualbox-guest-additions-iso:i386 (0 (null)) virtualbox-ose (0 (null)) libasound2 (0 (null)) libpulse0 (0 (null)) libsdl-ttf2.0-0 (0 (null)) kmod (16 (null)) kldutils (16 (null)) module-init-tools (0 (null)) linux-headers-generic (16 (null)) linux-headers-generic-pae (16 (null)) linux-headers-686-pae (16 (null)) linux-headers-amd64 (16 (null)) linux-headers-2.6-686 (16 (null)) linux-headers-2.6-amd64 (16 (null)) linux-headers (0 (null)) linux-image (0 (null)) gcc (0 (null)) make (16 (null)) build-essential (16 (null)) dpkg-dev (0 (null)) binutils (0 (null)) pdf-viewer (0 (null)) libgl1 (0 (null)) virtualbox (0 (null)) virtualbox:i386 (0 (null)) virtualbox-5.1:i386 (32 (null)) 

Provides: 

5.1.18-114002~Ubuntu~xenial - virtualbox (= ) 

From that we can see that all dependent packages are available through the default installation. No need to install any future package release.

You may have accidentally downloaded or installed a Virtual Box version built for Ubuntu 16.10 or another distribution. Please see this answer on how to add the Oracle repository to your sources and make sure you use the appropriate Ubuntu release for the sources line:

lsb_release -sc

In your /etc/apt/sources.list:
replace
'deb http://download.virtualbox.org/virtualbox/debian yakkety contrib'
by
'deb http://download.virtualbox.org/virtualbox/debian xenial contrib'

This worked for me:

1. sudo dpkg -i virtualbox-6.0_6.0.4-128413_Ubuntu_xenial_amd64.deb

2. then it shows Package libvpx3 is not installed..

3. sudo apt-get update.

4. sudo apt-get install libvpx3.