15.8. The city refuse collection department uses a fleet of small trucks that collect trash around the city and make periodic deliveries to four staging sites. Currently, two dump trucks transport the trash from these staging sites to an incinerator. One truck is assigned to service sites 1 and 2 and the other to sites 3 and 4. The net- work in the figure below gives the miles between the staging sites and the incin- erator, which is shown as node 0.