what is the correct value for the box that i got incorrect (what goes in place of the 15)?

this is the second time i am asking chegg this quetion

i have tried 30,20,45, and 10, and none of them work in place of 15 either

Return to questlon 6 Required Information Part 2 of 21 Trey Monson starts a merchandising business on December 1 and enters Into three Inventory purchases 071 Purchases on December 7 Purchases on December 14 Purchases on December 21 10 units e $ 6.00 cost 20 units@ S12.00 cost 15 units $14.00 cost points Required: Monson sells 15 units for $20 each on December 15. Assume the perlodic Inventory system is used. Determine the costs assigned to ending inventory when costs are assigned based on the welghted average method. (Amounts to be deducted should be Indicated with a minus sign. Round cost per unlts to 2 decimals.) Answer is not complete. Periodic Weighted Average Inventory on hand Cost of Goods Sold # of # of Cost Cost of Inventory Value Avg.Cost units units per unit Goods Sold per unit sold Purchase- December 7 10 6.00 60 Purchase- December 14 20O 12.00 240 Purchase-December 21 15 14.00 210 Available for Sale 45 11.33 510 December Sales 15 x 11.33 170 Total 60 S 680 0