please explain how you got the units for the second purchase

TB MC Qu. 05-163 (Algo) Salmone Company reported the following... Salmone Company reported the following purchases and sales of its only product. Salmone uses a periodic inventory system. Determine the cos assigned to cost of goods sold using FIFO. Date Activities Units Sold at Retail May 1 Beginning inventory Purchase Units Acquired at Cost 180 units @ $10-$1,800 250 units @ $12 = $3,000 May 5 May 10 Sales 170 units @ $20 May 15 Purchase. 130 units @ $13 = $1,690 May 24 Sales 120 units @ $21 Multiple Choice $3,120 O $3,610 $3,730