A company’s total cost for operating its two warehouses is C(x, y) = 1/9x² + 2x + y² + 5y + 100 dollars, where x represents the number of units stored at the first warehouse and y represents the number of units stored at the second. Find the average cost to store a unit if the first warehouse has between 40 and 80 units, and the second has between 30 and 70 units.