! Flight crew assignment model;
   MIN  2 X_1 + 3 X_2 + 4 X_3  + 6 X_4  + 7 X_5  + 5 X_6
      + 7 X_7 + 8 X_8 + 9 X_9 + 9 X_10 + 8 X_11 + 9 X_12
    Subject to
  ! Each flight must be covered
   X_1 + X_4 + X_7 + X_10 >= 1 
   X_2 + X_5 + X_8 + X_11 >= 1 
   X_3 + X_6 + X_9 + X_12 >= 1 
   X_4 + X_7 + X_9 + X_10 + X_12 >= 1 
   X_1 + X_6 + X_10 + X_11 >= 1 
   X_4 + X_5 + X_9 >= 1 
   X_7 + X_8 + X_10 + X_11 + X_12 >= 1 
   X_2 + X_4 + X_5 + X_9 >= 1 
   X_5 + X_8 + X_11 >= 1 
   X_3 + X_7 + X_8 + X_12 >= 1 
   X_6 + X_9 + X_10 + X_11 + X_12 >= 1 

 ! Must exactly use the crews we have;
   X_1 + X_2 + X_3 + X_4  + X_5  + X_6 
 + X_7 + X_8 + X_9 + X_10 + X_11 + X_12 = 3 
 END
! Make all the variables integer;
 INT 12
 