How to get out of a "...but it is not going to be installed"-type situation with apt-get?

[Kynn Jones <kynnjo@gmail.com>]

I apologize for all these apt-related questions.

I have rtfm'd on this ad nauseam.  For years, literally.

Still, I'm routinely rendered utterly helpless by this sort of thing:

    % sudo apt-get -qq update
    % sudo apt-get -qq upgrade
    % sudo apt-get -qq dist-upgrade
    % sudo apt-get install ffmpeg
    Reading package lists... Done
    Building dependency tree
    Reading state information... Done
    Some packages could not be installed. This may mean that you have
    requested an impossible situation or if you are using the unstable
    distribution that some required packages have not yet been created
    or been moved out of Incoming.
    The following information may help to resolve the situation:

    The following packages have unmet dependencies:
     ffmpeg : Depends: libavcodec-ffmpeg56 (>= 7:2.7) but it is not
going to be installed or
                       libavcodec-ffmpeg-extra56 (>= 7:2.7) but it is
not going to be installed
              Depends: libavdevice-ffmpeg56 (>= 7:2.6) but it is not
going to be installed
              Depends: libavfilter-ffmpeg5 (>= 7:2.4) but it is not
going to be installed
              Depends: libavformat-ffmpeg56 (>= 7:2.6) but it is not
going to be installed
    E: Unable to correct problems, you have held broken packages.


Where does one begin troubleshooting out such problems?

Thanks in advance for any help.

kj


PS:

    # /etc/apt/apt.conf
    APT::Default-Release "jessie";

    # /etc/apt/preferences
    Package: /^r-/
    Pin: release n=stretch
    Pin-Priority: 995

    # /etc/apt/sources.list
    deb http://http.us.debian.org/debian jessie main contrib non-free
    deb http://http.us.debian.org/debian jessie-updates main contrib
    deb http://security.debian.org jessie/updates main contrib non-free
    deb-src http://http.us.debian.org/debian jessie main contrib
    deb-src http://http.us.debian.org/debian jessie-updates main contrib
    deb-src http://security.debian.org jessie/updates main contrib non-free

    deb http://http.us.debian.org/debian stretch main contrib non-free
    deb-src http://http.us.debian.org/debian stretch main contrib

    deb http://download.videolan.org/pub/debian/stable /  # needed for
libdvdcss2 (see http://www.videolan.org/developers/libdvdcss.html)

I have made no changes to my /etc/apt/apt.conf.d/,
/etc/apt/preferences.d, and /etc/apt/sources.list.d/ that I'm aware
of.  The last two of these directories are empty.  The first one
contains

    % ls -l /etc/apt/apt.conf.d
    -rw-rw-r-- 1 root root   82 Nov 22 13:45 00CDMountPoint
    -rw-rw-r-- 1 root root   40 Nov 22 13:45 00trustcdrom
    -rw-rw-r-- 1 root root  643 Nov 22 13:45 01autoremove
    -rw-r--r-- 1 root root  981 Dec 19 09:44 01autoremove-kernels
    -rw-rw-r-- 1 root root  243 Nov 22 13:45 20dbus
    -rw-rw-r-- 1 root root  141 Nov 22 13:45 20listchanges
    -rw-rw-r-- 1 root root 1040 Nov 22 13:45 20packagekit
    -rw-rw-r-- 1 root root 3945 Nov 22 13:45 50unattended-upgrades
    -rw-rw-r-- 1 root root  182 Nov 22 13:45 70debconf