To decrease labor cost, a manufacturing facility recently changed from manual milling machines to CNC milling machines for all its production. It finds that an operator takes $1$ minute to travel between machines, $4$ minutes to load a machine, $2$ minutes to unload a machine, $20$ minutes to machine a part, $3$ minutes to inspect and pack a finished part. The labor cost is $$50/$hr per person and the machine cost is $$25/$hr per machine. $(40$ pts$)$