1. In the five-warehouse solution, even though the Columbus warehouse is very close to the plants, it still chooses to serve customers from additional warehouses. Why? In other words, why not save on inbound transportation and serve most customers from Columbus?

Here, the inbound transportation is the expensive leg. We want to minimize the total cost. To do this, we need to minimize both inbound and outbound. Locating a single facility in Columbus minimizes the inbound transportation, but the outbound would be expensive.