A cylindrical riser is to be designed for a sand casting mold. The length of the cylinder is to be 1.5 times its diameter. The casting (part) is a square plate, each side = 11 in and thickness = 0.5 in. If the metal is cast iron, and the mold constant = 16.0 min/in² in Chvorinov's rule (n=2), determine the dimensions (diameter D and height H) of the riser so that it will take 25% longer for the riser to solidify.