A 1125 ft³ open-top rectangular tank with a square base x ft on a side and y ft deep is to be built with its top flush with the ground to catch runoff water. The costs associated with the tank involve not only the material from which the tank is made but also an excavation charge proportional to the product xy.

a. If the total cost is c = 5(x² + 4xy) + 10xy, what values of x and y will minimize it?

b. Give a possible scenario for the cost function in part (a).