
  (solve) : right: auto
  +
      0px : margin-right
  +
      0px : border-right-width
  +
      0px : padding-right
  +
  (based on the content) : width: auto
  +
      0px : padding-left
  +
      0px : border-left-width
  +
      0px : margin-left
  +
     80px : left: auto
    =====================
    320px : width of containing block

gives us:

  (solve) : right: auto
  +
      0px : margin-right
  +
      0px : border-right-width
  +
      0px : padding-right
  +
     80px : (based on the content)