The state government has one million dollars available to divide between the construction of day-care centers, which cost $20,000 per center, and the re- pavement of roads, which costs $50,000 per mile. Because all the money not spent on road work is used for day-care centers, the number of centers funded, c, depends on the number of miles of roads paved, r. a, Express the number of centers funded, c, as a linear function of r, the number of miles of road paved