A company manufactures a product at two plants, one in Dallas and one in Kansas City. The Dallas plant can produce up to 200 units per day, and the Kansas City plant can produce up to 250 units per day. The product is shipped by air to customers in San Francisco and Washington, D.C. The customers in each city require 225 units of the product per day. The company believes that it might be cheaper to ship some units of the product through Denver or Atlanta on the way to their final destination. The cost of transporting a unit of the product between each pair of cities is given in the file 3.xls. Given that this company wants to minimize the total cost of shipping the required units to the customers, solve a linear programming model for this network flow problem.