Show me the steps to solve this using solver on excel. Toyco produces a line of Bonnie dolls and accessories at its plants in New York and Baltimore that must be shipped to distribution centers in Chicago and Los Angeles. The company uses Air Freight, Inc., to make its shipments. Suppose that it can ship directly or through Pittsburgh and Denver. The daily production rates at the plants are respectively $5,000$ and $7,000$ units daily, and the demands at the distribution centers are respectively $3,500$ and $8,500$ units daily. The costs of shipping $1,000$ units are given in the following table. Find the optimal shipping routes and the associated cost.