Partridge bookstore had 500 units on hand at january 1, costing $9 each. purchases and sales during the month of january were as follows: partridge uses the periodic inventory system. according to a physical count, 365 units were on hand at january 31. the cost of the inventory at january 31, under the fifo method is a a : $4,015. b : $3,650. c : $3,900. d : $3,285.