solve using schematic diagram tree

9.15 A fast-food chain wants to build four stores in the Chicago area. In the past, the chain has used six different construction companies, and, having been satisfied with each, has invited each to bid on each job. The final bids (in thousands of dollars) were as shown in Table 9-3. Since the fast-food chain wants to have each of the new stores ready as quickly as possible, it will award at most one job to a construction company. What assignment results in minimum total cost to the fast-food chain