The Ace Battery Company has forecast its sales in units as follows: January 2,800 May 3,350 February 2,650 June 3,500 March 2,600 July 3,200 April 3,100 Ace always keeps an ending inventory equal to 140 percent of the next month's expected sales. The ending inventory for December (January's beginning inventory) is 3,920 units, which is consistent with this policy. Materials cost $11 per unit and are paid for in the month after production. Labour cost is $4 per unit and is paid in the month the cost is incurred. Overhead costs are $16,000 per month.