A solid right circular cylinder of radius 8cm and height 15cm is completely melted and recast into identical solid spheres, each of radius 3cm.
a. (i) Calculate the volume of the cylinder.
b. (ii) Determine the greatest possible number of complete spheres that can be made.
c. (iii) Find the volume of metal, if any, that will remain unused after casting the maximum possible number of spheres.