$6-25$: Everett Mann$$s Dream Store sells waterbeds and supplies. The best$-$selling bed in the store has an annual demand of $400$ units. The ordering cost is $$40,$ while the holding cost is $$5$ per unit per year. There are $250$ working days per year, and the lead$-$time is $6$ days. To minimize total cost, how many units should be ordered each time an order is placed? If the holding cost per unit was $$6$ instead of $$5,$ what would the optimal order quantity be$?$