min:  6 x11 +  8 x12  + 9 x13 + 6  x14
   +  5 x21 + 10 x22 +  3 x23 + 7  x24
   +  3 x31 +  9 x32 +  2 x33 + 4  x34;
 x11 + x12 + x13 + x14 = 7;
 x21 + x22 + x23 + x24 = 10;
 x31 + x32 + x33 + x34 = 13;
 x11 + x21 + x31 = 5;
 x12 + x22 + x32 = 10;
 x13 + x23 + x33 =  9;
 x14 + x24 + x34 =  6;