> eqn1 := x1 = 400;

                           eqn1 := x1 = 400

> eqn2 := -x1 + x2 + x5 = 300;

                     eqn2 := -x1 + x2 + x5 = 300

> eqn3 := x2 + x3 = 800;

                        eqn3 := x2 + x3 = 800

> eqn4 := x4 + x5 = 500;

                        eqn4 := x4 + x5 = 500

> eqn5 := x3 + x4 = 600;

                        eqn5 := x3 + x4 = 600

> solve({eqn1, eqn2, eqn3, eqn4, eqn5}, {x1, x2, x3, x4, x5});

  {x4 = -x5 + 500, x1 = 400, x2 = 700 - x5, x3 = x5 + 100, x5 = x5}

>