diff --git a/04_TransportProblem.ipynb b/04_TransportProblem.ipynb
index 6650e2d..ae1eef0 100644
--- a/04_TransportProblem.ipynb
+++ b/04_TransportProblem.ipynb
@@ -303,8 +303,11 @@
     "#  cost ..        z  =e=  sum((i,j), c(i,j)*x(i,j)) ;\n",
     "#  Model transport /all/ ;\n",
     "#  Solve transport using lp minimizing z ;\n",
+    "#\n",
+    "# itertools.product() returns the Cartesian product of two or more iterables\n\n",
+    "import itertools\n",
     "def objective_rule(model):\n",
-    "    return sum(model.c[i,j]*model.x[i,j] for i in model.i for j in model.j)\n",
+    "    return sum(model.c[i,j]*model.x[i,j] for i, j in itertools.product(model.i, model.j))\n",
     "\n",
     "model.objective = Objective(rule=objective_rule, sense=minimize, doc='Define objective function')"
    ]