13. An industry has three assembly plants for scooters at cities 1 , 2 and 3. The scooters are shipped to retail customers 6,7 and 8 from storage centers located at cities 4 and 5 . The shipping cost per scooter (in \$100) are shown on the arcs of the following network. (a) Solve the problem as transshipment model, and find the optimal solution. (b) Suppose that the distribution centre 4 can sell 200 scooters in the local market directly to the customers. Find the new optimal solution