7 Use branch-and-bound to determine a way (if any exists to place four queens on a 4 4 chessboard so that no queen can capture another queen. (Hint: Let xij  1 if a queen is placed in row i and column j of the chessboard and xij  0 otherwise. Then branch as in the machine-delay problem.

Many nodes may be eliminated from consideration because they are infeasible. For example, the node associated with the arcs x11  x22  1 is infeasible, because the two queens can capture each other.)