min:
      25 x_a1 + 12 x_a2 + 28 x_a3 + 35 x_a4
    + 33 x_b1 + 18 x_b2 + 38 x_b3 + 42 x_b4
    + 34 x_c1 + 16 x_c2 + 21 x_c3 + 26 x_c4;


sa: x_a1 + x_a2 + x_a3 + x_a4 = 72;
sb: x_b1 + x_b2 + x_b3 + x_b4 = 53;
sc: x_c1 + x_c2 + x_c3 + x_c4 = 48;

d1: x_a1 + x_b1 + x_c1 = 52;
d2: x_a2 + x_b2 + x_c2 = 38;
d3: x_a3 + x_b3 + x_c3 = 29;
d4: x_a4 + x_b4 + x_c4 = 54;