A supplier offers a manufacturer an item at a 5% discount on orders of 500 units or more, and a 15% discount on orders of 1,000 units or more. From past experience, it appears that demand for this item is 3,075 units/year. If the ordering cost is $50, the basic price (without discount) of the item is $2, and an r value of 0.20 $/$/year is used, how often should the manufacturer order from the supplier? What order quantity should be used?