Simultaneous Equation Solver?

Where can I find an equation solver that can solve simultaneous equations with height 49 variables? Or how can I make myself that will do that? Note: I prefer it to be free. I do not want to buy some or Mathematica something like that.

Octave is free. it is similar to MATLAB. My TI-83 Calculator accepts a 50×50 matrix for consequent reduction. I do not know How big can it go, though.

Using Mathematica to Solve MIMO Control Problems