help please! i just need to understand the math part!

USE THE FOLLOWING INFORMATION TO ANSWER QUESTIONS 3 - 5 The following data represent the beginning inventory and, in the order of occurrence, the purchases and sales of Mullenax Company for an operating period. Units Unit Cost Total Cost Units Sold Beginning Inventory 20 $87 $ 1.740 Purchase No. 1 20 93 1,860 Sale No. 1 30 Purchase No. 2 90 96 8,640 Sale No. 2 80 Purchase No.3 70 108 7.560 Totals 200 $19,800 110 3. Assuming Mullenax Company uses FIFO periodic inventory system, the ending inventory cost is: A) $9,720 B) $8,400 C) $9,480 D) $7,560 4. Assuming Mullenax Company uses LIFO periodic inventory procedures, the ending inventory cost is: A) $7,380 B) $9,480 C) $9,390 D) $8,400 5. Assuming Mullenax Company uses weighted-average (periodic) inventory procedures, the ending inventory cost is: A) $ 8,910 B) $ 8,640 C) $ 9,468 D) $10,890