Nelson Manufacturing uses a process costing system with $2$ departments, Assembly and Packaging. Units in process in the Assembly Department at the beginning of the period totaled $2,000.$ During the period, an additional $15,000$ units were started, and by the end of the period, only $6,000$ unfinished units remained. $(1)$ During the period,

units must have been transferred to the Packaging Department. $(2)$ Units started and completed during the period in the Assembly Department must have totaled

$.$