Rocky Mountain Tire Center sells 7,000 go-cart tires per year. The ordering cost for each order is 540, and the holding cost is 50% of the purchase price of the tires per year. The purchase price is $21 per tire if fewer than 200 tires are ordered, $16 per tire if 200 or more, but fewer than 5.000, tires are ordered, and $15 per tire if 5,000 or more tires are ordered. a) How many tires should Rocky Mountain order each time it places an order?