The following units of a particular item was purchased and sold during the period: Beginning inventory of 10 units @ $5; First purchase of 15 units @ $6; Sales of 10 units; Second purchase 10 units @ $7; Sale of 8 units; Third purchase 15 units @ $8; Sale of 17 units What is the total cost of the 15 units on hand (held in inventory) at the end of the period, as determined under the periodic inventory system by the lifo costing method? 80 $90 $100 $120