A store sells shoes at an approximate constant rate of 500 pairs of shoes every 3 months. The current buying policy is to order 500 pairs each time an order is placed . It costs 30 bath to place an order. The annual holding cost rate is 20% . With the order quanity of 500, the store obtains the shoes at the lowest possible unit cost of 28 bath per pair . Other quantity discounts offered by the manufacturer are as follows