3. ABC company uses 8,000 pens every year. For each pen, each order cost is $30 and the annual holding cost is 10 per cent of unit cost. The unit cost depends on the quantity ordered as follows: 18 Quantity Unit Cost (5) 1-199 20 200-499 500-799 800+ Assume that demand has a constant rate and do not stockout; a) What is the optimal order size? (10 points) b) Determine the yearly order quantity (5 points) 16 12