Rocky Mountain Tire Center sells 15,000 tires of a particular type per year. The ordering cost for each order is $30, and the holding cost is 20% of the purchase price of the tires per year. The purchase price is $20 per tire if fewer than 500 tires are ordered, $18 per tire if 500 or more - but fewer than 1,000 tires are ordered, and $17 per tire if 1,000 or more tires are ordered. How many tires should Rocky Mountain order each time it places an order?