of cell in the middle column
 +
   30px : half of border-right since such border must be split with cell in 3rd column

  =======
  140px : width of the middle column


  So,
      400px : table set width
    -
      140px : width of the middle column
      ======
      260px

      So, each of the 2 remaining columns must be
      half of such extra horizontal table space,
      which is 260px divided by 2 == 130px.

  1st column width calculations
  -----------------------------

    (solve) : width of ce