data3.txt for solving Ax=b by Gaussian Elimination and Back Substitution
  5
  10.0   1.0   2.0   3.0   4.0
   1.0   9.0  -1.0   2.0  -3.0
   2.0  -1.0   7.0   3.0  -5.0
   3.0   2.0   3.0  12.0  -1.0
   4.0  -3.0  -5.0  -1.0  15.0
  12.0 -27.0  14.0 -17.0  12.0
   1.0  -2.0   3.0  -2.0   1.0