A new wireless carrier has decided to enter the Pacific Northwest cell phone market. Before entering the market, though, it needs to construct cell phone towers to ensure that each zip code in the Pacific Northwest will be covered. The carrier is considering a set of locations on which to build cell towers. For each candidate location l, the carrier has determined which zip codes could be covered if a cell phone tower was built at that location, as well as the cost of building a tower at that location. How can the carrier minimize the cost of entering the Pacific Northwest market?

What is the formulation to minimize the cost ?