Using ASCIIMath, how can we align variables in proper order in a system of linear equations

Question

For instance how can we display folliwing in MathJax using ASCIIMath:

Answer 1

This is as close as I could come:

{
  [      -x_1, ,   , ,   ,+,x_4,=,\qquad 130],
  [          , ,   , ,x_3,-,x_4,=,\qquad\quad 60],
  [          , ,x_2,-,x_3, ,   ,=,-240],
  [\qquad x_1,-,x_2, ,   , ,   ,=,\qquad\quad 50]
:}

The spacing is a bit ad hoc, but perhaps it is good enough for you.