The Carter Caterer Company must have the following number of clean napkins available at the beginning of each of the next four days: day 115; day 212; day 318; day 46. After being used, a napkin can be cleaned by one of two methods: fast service or slow service. Fast service costs 10 per napkin, and a napkin cleaned via fast service is available for use the day after it is last used. Slow service costs 6 per napkin, and these napkins can be reused two days after they are last used. New napkins can be purchased for a cost of 20 per napkin. Formulate minimum cost network flow problem, show nodes too